pm2fb: Permedia 2V memory clock setting

Permedia 2V uses its own registers to set a memory clock. The
patch adds these registers and uses them in the set_memclock()
function.

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>