drivers: pl011: add SBSA mode

ARM Server Base System Architecture defines Generic UART interface,
which is subset of PL011 UART.

Minimal SBSA UART implementation does not define UART hardware
configuration registers. Basically, only FIFOs and interrupt management
operation are defined.

Add SBSA mode to PL011 UART driver, so it can be used at SBSA-compatible
platforms, like Xen guest.

Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>