soc/intel/common: irq: Use correct size_t length modifier

Building an image for the Purism Mini v2 with `x86_64-linux-gnu-gcc-11`
fails with the format warning below as the size of size_t differs
between 32-bit and 64-bit.

        CC         ramstage/soc/intel/common/block/irq/irq.o
    src/soc/intel/common/block/irq/irq.c: In function 'assign_fixed_pirqs':
    src/soc/intel/common/block/irq/irq.c:186:90: error: format '%lu' expects argument of type 'long unsigned int', but argument 5 has type 'size_t' {aka 'unsigned int'} [-Werror=format=]
      186 |                         printk(BIOS_ERR, "ERROR: Slot %u, pirq %u, no pin for function %lu\n",
          |                                                                                        ~~^
          |                                                                                          |
          |                                                                                          long unsigned int
          |                                                                                        %u
      187 |                                constraints->slot, fixed_pirq, i);
          |                                                               ~
          |                                                               |
          |                                                               size_t {aka unsigned int}
        CC         ramstage/soc/intel/common/block/gspi/gspi.o
        CC         ramstage/soc/intel/common/block/graphics/graphics.o
        CC         ramstage/soc/intel/common/block/gpio/gpio.o
        CC         ramstage/soc/intel/common/block/gpio/gpio_dev.o

The variable `i` is of type size_t, so use the corresponding length
modifier `z`.

Fixes: b59980b54e ("soc/intel/common: Add new IRQ module")
Change-Id: I09f4a8d22a2964471344f5dcf971dfa801555f4a
Signed-off-by: Paul Menzel <pmenzel@molgen.mpg.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/59056
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>

src/soc/intel/common/block/irq/irq.c

@@ -183,7 +183,7 @@ static bool assign_fixed_pirqs(const struct slot_irq_constraints *constraints,
 		   fixed pin */
 		const enum pci_pin pin = fn_pin_map[i];
 		if (pin == PCI_INT_NONE) {
-			printk(BIOS_ERR, "ERROR: Slot %u, pirq %u, no pin for function %lu\n",
+			printk(BIOS_ERR, "ERROR: Slot %u, pirq %u, no pin for function %zu\n",
 			       constraints->slot, fixed_pirq, i);
 			return false;
 		}