regmap: mmio: Staticize regmap_mmio_gen_context()

regmap_mmio_gen_context() is only used in regmap-mmio.c. Thus make it static.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>