arm: v7: Kconfig: Introduce SYS_ARM_CACHE_CP15

Certain ARM architectures like ARMv7-A, ARMv7-R has support for
enabling caches using CP15 registers. To have a common support
for all these architectures, introduce a Kconfig symbol
SYS_ARM_CACHE_CP15 that selects cache-cp15.c

Tested-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>