mfd: syscon: Add arguments support for syscon reference

There are a lot of similar global registers being used across multiple SoCs
from Unisoc. But most of these registers are assigned with different offset
for different SoCs. It is hard to handle all of them in an all-in-one
kernel image.

Add a helper function to get regmap with arguments where we could put some
extra information such as the offset value.

Signed-off-by: Orson Zhai <orson.zhai@unisoc.com>
Tested-by: Baolin Wang <baolin.wang@unisoc.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>