regcache: flat: Introduce register strider order

Here we introduce regcache_flat_get_index(), which using register
stride order and bit rotation, will save some memory spaces for
flat cache. Though this will also lost some access performance,
since the bit rotation is used to get the index of the cache array,
and this could be ingored for memory I/O accessing.

Signed-off-by: Xiubo Li <lixiubo@cmss.chinamobile.com>
Signed-off-by: Mark Brown <broonie@kernel.org>