video: ARM CLCD: Move registers to a separate header.

We'd like to reuse these register definitions for the DRM CLCD driver,
but there's a bunch of fbdev-specific code in the current header.

v2: Add #ifndef guard.

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: http://patchwork.freedesktop.org/patch/msgid/20170413031746.12921-1-eric@anholt.net