regmap: Cache single values read from the chip

If we don't have a cached value for a register and we can cache it then
when we do a read a value we should add it to the cache to save rereading
it later on. Do this for single register reads, for block reads the code
would be a little more complex and this covers most practical usage.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>