[ARM] Allow Versatile to be built for AB and PB

If a configuration was chosen to support both the Versatile
AB and PB boards, the result would write to registers not
available on the PB version of the board.  Resolve this by
using machine_is_xxx().

Also, for the CLCD, despite how the code looks, both the AB
and PB access the same location to control the clock rate -
it's just called something different between the two board
versions.  Invent our own name for this location and use it
unconditionally.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>