arm: serial: Add debug UART capability to the pl01x driver

This patch adds an ability to use pl01x as a debug UART. It must
be configured like other types of debug UARTs

Signed-off-by: Sergey Temerkhanov <s.temerkhanov@gmail.com>
Signed-off-by: Radha Mohan Chintakuntla <rchintakuntla@cavium.com>
[trini: Update for _debug_uart_init change]
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>