dt-bindings: pwm-qti-lpg: Update reg definition

Commit f6ea1a19 ("ARM: dts: msm: add PMIC devices for Lahaina")
specifies the #address-cells and #size-cells of peripheral devices under
PMIC device nodes on the SPMI bus as 1 and 0 respectively. This requires
that the reg property specification of all child nodes of the PMIC
devices be updated.

Update pwm-qti-lpg bindings accordingly. Also add missing required
property to example.

Change-Id: I0b0fc8fdaa58f03904cb82237d7ed56e37512aa1