regmap: cache: Sort include headers alphabetically

If the inlcude headers aren't sorted alphabetically, then the
logical choice is to append new ones, however that creates a
lot of potential for conflicts or duplicates because every change
will then add new includes in the same location.

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