regulator: mem-acc-regulator: add support for arbitrary ACC selector sizes

Add support for arbitrary L1 and L2 memory accelerator selector
sizes.  The default value of 2 is not sufficient for all targets.
Provide this support via a pair of new device tree properties.

Change-Id: I8c625e16852451be500c711c4011971fd7dfce34
Signed-off-by: David Collins <collinsd@codeaurora.org>