regmap: Add multi register write support for soundwire

regcache sync can call multi register write to sync multiple
register writes to hardware in one transaction. Change enables
support from soundwire framework to sync multiple registers
in one transaction.

Change-Id: Iafe35bf9b8987fb7214efff0d7d4ae3e0a6a4072
Signed-off-by: Sudheer Papothi <spapothi@codeaurora.org>