acpi/acpi.c: Assign coreboot_rsdp for QEMU

At present coreboot_rsdp remains unset for QEMU, which results in
an incomplete LB_TAG_ACPI_RSDP coreboot table generated.

Fix this by assigning coreboot_rsdp properly.

TEST=Build coreboot for QEMU x86 i440fx (default) with U-Boot x86
as the payload, boot coreboot.rom with QEMU, and run 'acpi list'
from U-Boot shell to show the ACPI tables.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Change-Id: I5bc3f0528d4431fd388ca52b8865f9be0e1faf92
Reviewed-on: https://review.coreboot.org/c/coreboot/+/75088
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Reviewed-by: Tim Wawrzynczak <inforichland@gmail.com>

src/acpi/acpi.c

@@ -1857,6 +1857,7 @@ unsigned long write_acpi_tables(unsigned long start)
 		for (void *p = (void *)current; p < (void *)fw; p += 16) {
 			if (valid_rsdp((acpi_rsdp_t *)p)) {
 				rsdp = p;
+				coreboot_rsdp = (uintptr_t)rsdp;
 				break;
 			}
 		}