regulator: rpmh-regulator: initiate child device probe

There can be child devices for a rpmh-regulator resource, which
have to be probed after the rpmh-regulator resource probes
successfully. Call of_platform_populate to initiate the probing
of child devices.

Change-Id: If45613e2e15a2179a62c1eb94a915407d80aff6d
Signed-off-by: Ram Chandrasekar <rkumbako@codeaurora.org>