[ARM] 3366/1: Allow the 16bpp mode configuration in the CLCD control register

Patch from Catalin Marinas

Starting with PL111, the 5551 or 565 modes can be configured in the
primecell's control register directly. This patch detects the required mode
and sets the correct value.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>