arm: serial: Add Kconfig entries to facilitate usage of the pl01x driver for early debug output

This patch adds Kconfig entries to facilitate usage of pl01x as
a debug UART.

Signed-off-by: Sergey Temerkhanov <s.temerkhanov@gmail.com>
Signed-off-by: Radha Mohan Chintakuntla <rchintakuntla@cavium.com>

Acked-by: Simon Glass <sjg@chromium.org>