drivers: mfd: Add support for slimbus multi register write

slimbus driver supports writing multiple registers with a single
header. Add an API in core driver to support slimbus
multi register write. This will help in reducing the cold start
latency.

Change-Id: I158ef10e5a148408d9f4268da388eac515e46433
Signed-off-by: Sudheer Papothi <spapothi@codeaurora.org>