rpi: Allow to boot without serial

When we enable CONFIG_OF_BOARD on Raspberry Pis, we may end up without
serial console support in early boot. Hence we need to make the serial
port optional, otherwise we will never get to the point where serial
would be probed.

Signed-off-by: Alexander Graf <agraf@suse.de>

configs/rpi_0_w_defconfig

@@ -34,3 +34,4 @@ CONFIG_SYS_WHITE_ON_BLACK=y
 CONFIG_CONSOLE_SCROLL_LINES=10
 CONFIG_PHYS_TO_BUS=y
 CONFIG_OF_LIBFDT_OVERLAY=y
+# CONFIG_REQUIRE_SERIAL_CONSOLE is not set

configs/rpi_2_defconfig

@@ -34,3 +34,4 @@ CONFIG_SYS_WHITE_ON_BLACK=y
 CONFIG_CONSOLE_SCROLL_LINES=10
 CONFIG_PHYS_TO_BUS=y
 CONFIG_OF_LIBFDT_OVERLAY=y
+# CONFIG_REQUIRE_SERIAL_CONSOLE is not set

configs/rpi_defconfig

@@ -34,3 +34,4 @@ CONFIG_SYS_WHITE_ON_BLACK=y
 CONFIG_CONSOLE_SCROLL_LINES=10
 CONFIG_PHYS_TO_BUS=y
 CONFIG_OF_LIBFDT_OVERLAY=y
+# CONFIG_REQUIRE_SERIAL_CONSOLE is not set