ep93xx: Move PHYS_BASE defines to local SoC header file

The PHYS_BASE defines in arch/arm/mach-ep93xx/include/mach-ep93xx-regs.h are
only used in the SoC code. Move the defines to a local header file.

Signed-off-by: Ryan Mallon <rmallon@gmail.com>
Reviewed-by: Mika Westerberg <mika.westerberg@iki.fi>
Acked-by: Hartley Sweeten <hsweeten@visionengravers.com>