dt-bindings: smb1390-psy: Add property to support dual charge pumps

Dual SMB1390 PMICs, master and slave, are supported as companion chargers
to the primary charge PMIC. The CP driver is split into master and slave
drivers, with slave mirroring master's operations. Add property to support
dual charge pumps.

Change-Id: I5b7f5962afd84243c0269aa56586bb75030e27ed