regmap: allow const array for {devm_,}regmap_field_bulk_alloc reg_fields

regmap: allow const array for {devm_,}regmap_field_bulk_alloc reg_fields

The reg_fields array fed to {devm_}regmap_field_bulk_alloc is currently
not const, which is not correct on semantics (the functions shouldn't
change reg_field contents) and prevents pre-defined const reg_field
array to be used.

As the implementation of this function doesn't change the content of it,
just add const to its prototype.

Signed-off-by: Icenowy Zheng <icenowy@sipeed.com>
Link: https://lore.kernel.org/r/20210802063741.76301-1-icenowy@sipeed.com
Signed-off-by: Mark Brown <broonie@kernel.org>