ARM: imx: gpc: Fix the imx_gpc_genpd_init() error path

If of_genpd_add_provider_onecell() fails the following kernel crash is
observed on a kernel built with multi_v7_defconfig:

[    1.739301] [00000040] *pgd=00000000
[    1.739310] Internal error: Oops: 5 [#1] SMP ARM
[    1.739319] Modules linked in:
[    1.739328] CPU: 1 PID: 95 Comm: kworker/1:4 Not tainted 4.8.0-11897-g6b5e09a #1
[    1.739331] Hardware name: Freescale i.MX6 Quad/DualLite (Device Tree)
[    1.739352] Workqueue: pm genpd_power_off_work_fn
[    1.739356] task: ee63d400 task.stack: ee70a000
[    1.739365] PC is at mutex_lock+0xc/0x4c
[    1.739374] LR is at regulator_disable+0x2c/0x60
[    1.739379] pc : [<c0bc0da0>]    lr : [<c06e4b10>]    psr: 60000013
[    1.739379] sp : ee70beb0  ip : 10624dd3  fp : ee6e6280
[    1.739382] r10: eefb0900  r9 : 00000000  r8 : c1309918
[    1.739385] r7 : 00000000  r6 : 00000040  r5 : 00000000  r4 : 00000040
[    1.739390] r3 : 0000004c  r2 : 7fffd540  r1 : 000001e4  r0 : 00000040

Instead of returning of_genpd_add_provider_onecell() directly,
we should check its return value and in the case of error we
should unwind the previously taken actions, which in these case are:
- Call imx6q_pm_pu_power_off()
- Set imx6q_pu_domain.reg back to NULL

Setting imx6q_pu_domain.reg to NULL in the error case is important
as it will prevent further operations in the pu_reg regulator.

This kernel crash is not observed with imx_v6_v7_defconfig because
it selects GPU and VPU drivers, which are consumers of the GPC block
and thus change the refcount of the pu_reg regulator.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>