ARM: pxa: pxa95x is incompatible with earlier pxa

We cannot support ARMv5 and ARMv7 based boards in a single kernel,
so introduce a new option in mach-pxa to select between the two.

The PJ4 (ARMv7) based boards are now only visible when
CONFIG_ARCH_PXA_V7 is set, the other boards are only visible
when it's not set.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>