firmware: arm_scmi: pm: Propagate return value to caller

of_genpd_add_provider_onecell may return error, so let's propagate
its return value to caller

Link: https://lore.kernel.org/r/20211116064227.20571-1-peng.fan@oss.nxp.com
Fixes: 898216c9

 ("firmware: arm_scmi: add device power domain support using genpd")
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>