ARM: ep93xx: make mach/ep93xx-regs.h local

Nothing relies on it outside of arch/arm/mach-ep93xx/, so just move
it there.

Cc: Hartley Sweeten <hsweeten@visionengravers.com>
Tested-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>