soc: imx: gpcv2: fix regulator deferred probe

soc: imx: gpcv2: fix regulator deferred probe

If a regulator requests a deferred probe, the power domain gets
initialized twice. This leads to a list double add (without
list debugging the kernel hangs due to the double add later):

  WARNING: CPU: 0 PID: 19 at lib/list_debug.c:31 __list_add_valid+0xbc/0xc4
  list_add double add: new=c1229754, prev=c12383b4, next=c1229754.

Initialize the power domain after we get the regulator, and do
not print an error in case the regulator deferrs probing.

Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Andrey Smirnov <andrew.smirnov@gmail.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Fixes: 03aa12629fc4 ("soc: imx: Add GPCv2 power gating driver")
Signed-off-by: Stefan Agner <stefan@agner.ch>