simplefb: formalize pseudo palette handling

Signed-off-by: Luc Verhaegen <libv@skynet.be>
simplefb: add goto error path to probe

Signed-off-by: Luc Verhaegen <libv@skynet.be>
simplefb: add clock handling code

This claims and enables clocks listed in the simple framebuffer dt node.
This is needed so that the display engine, in case the required clocks
are known by the kernel code and are described in the dt, will remain
properly enabled.

Signed-off-by: Luc Verhaegen <libv@skynet.be>