soc/intel/apollolake: implement bootblock_soc_early_init()

Provide a bootblock_soc_early_init() to that takes care of
initializing the UART on behalf of the mainboard when serial
console is enabled.

Change-Id: I2d3875110b6f58a9e0b4c113084b85817aa05a87
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/13793
Tested-by: build bot (Jenkins)
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Andrey Petrov <andrey.petrov@intel.com>

src/soc/intel/apollolake/bootblock/bootblock.c

@@ -17,6 +17,7 @@
 #include <soc/cpu.h>
 #include <soc/northbridge.h>
 #include <soc/pci_devs.h>
+#include <soc/uart.h>
 
 void asmlinkage bootblock_c_entry(void)
 {
@@ -42,3 +43,10 @@ void platform_prog_run(struct prog *prog)
 	msr.lo |= (1 << 8);
 	wrmsr(MSR_POWER_MISC, msr);
 }
+
+void bootblock_soc_early_init(void)
+{
+	/* Prepare UART for serial console. */
+	if (IS_ENABLED(CONFIG_SOC_UART_DEBUG))
+		soc_console_uart_init();
+}