configs: split ti_armv7_common into a omap generic header

TI armv7 based SoCs are based on two architectures - one based on OMAP
generation architecture and others based on Keystone architecture.

Many of the options are architecture specific, however a lot are common
with v7 architecture. So, step 1 will be to move out OMAP specific stuff
from ti_armv7_common into a ti_armv7_omap.h header which is then used
by all the relevant architecture headers.

Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Nishanth Menon <nm@ti.com>