soc/amd/piasso/uart: move get_uart_base prototype to common code header

This will result in less code duplication when the common AMD SoC UART
support gets used for more AMD SoCs.

Change-Id: Id1786f32324de3e3947d792c599e2019705c5a85
Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/49373
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Marshall Dawson <marshalldawson3rd@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/amd/picasso/uart.c

@@ -6,6 +6,7 @@
 #include <device/mmio.h>
 #include <amdblocks/gpio_banks.h>
 #include <amdblocks/aoac.h>
+#include <amdblocks/uart.h>
 #include <soc/southbridge.h>
 #include <soc/gpio.h>
 #include <soc/uart.h>