power: smb350_charger: add support for specifying fuel gauge part name

SMB350 charger needs to use external fuel gauge part to get battery
capacity. Hence add device support for specifying the name of the fuel
gauge part.

Change-Id: Iba1c48e99b1a602b1e2787690e8294f14a8724d9
Signed-off-by: Ke Liu <keliu@codeaurora.org>