mfd: Fix TWL4030 build on some ARM variants

Many ARM platforms do not provide a mach/cpu.h so rather than guarding
the use of that header with CONFIG_ARM guard it with the guards used
when testing for the OMAP variants in the body of the code.

Signed-off-by: Mark Brown <broonie@sirena.org.uk>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Samuel Ortiz <sameo@openedhand.com>