drivers: PL011: allow to supply fixed option string

The SBSA UART has a fixed baud rate and flow control setting, which
cannot be changed or queried by software.
Add a vendor specific property to always return fixed values when
trying to read the console options.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Tested-by: Mark Langsdorf <mlangsdo@redhat.com>
Tested-by: Naresh Bhat <nbhat@cavium.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>