mfd: altera-a10sr: Make altr_a10sr_regmap_config static const

It's only used in this driver and never get modified, make it static const.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Reviewed-by: Thor Thayer <tthayer@opensource.altera.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>