Serial: p1011: new vendor init options

Two new options:

CONFIG_PL011_SERIAL_RLCR

Some vendor versions of PL011 serial ports (e.g. ST-Ericsson U8500)
have separate receive and transmit line control registers.  Set
this variable to initialize the extra register.

CONFIG_PL011_SERIAL_FLUSH_ON_INIT

On some platforms (e.g. U8500) U-Boot is loaded by a second stage
boot loader that has already initialized the UART.  Define this
variable to flush the UART at init time.
empty fifo on init

Signed-off-by: John Rigby <john.rigby@linaro.org>
Signed-off-by: Rabin Vincent <rabin.vincent@stericsson.com>