ARM: simplify machdirs/platdirs handling

There is only one plat-* directory left, and the MACHINE variable is
only used for the mach/*.h header path.

Simplify this by removing the checks for ARCH_MULTIPLATFORM and
ARM_SINGLE_ARMV7M, and just adding the include directories for the
remaining three platforms manually.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>