boot: zephyr: add Kconfig for arm cortex-m that implements a cache

The Cache is an optional configuration of both the ARM Cortex-M7 and
Cortex-M55. Previously, it was just checking that it was just an M7
rather than knowing that the CPU actually was built with the cache.

Signed-off-by: Ryan McClelland <ryanmcclelland@fb.com>
Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>