On the Versatile PB, PL110 graphics adaptor only natively supports
5551 pixel format; an external mux swaps bits around to allow
RGB565 and BGR565, under the control of bits [1:0] in the SYS_CLCD
system register.

Implement these SYS_CLCD register bits, and use a gpio line to
feed them out of the system register model, across the versatilepb
board and into the pl110 so we can select the right format.

This is necessary as recent Linux versatile kernels default to
programming the CLCD and mux for 16 bit BGR rather than 16 bit RGB.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>