pinctrl: iproc: Fix iProc and NSP GPIO support

Since commit 44a7185c ("of/platform: Add common method to populate
default bus"), ARM64 platform devices are populated at the
arch_initcall_sync level; as a result, the platform_driver_probe calls
in both the iProc and NSP GPIO drivers fail with -ENODEV since by that
time the platform device was not yet registered.

Replace platform_driver_probe with platform_driver_register, that allow
the device to be register later

Fixes: 44a7185c ("of/platform: Add common method to populate default bus")
Signed-off-by: Ray Jui <ray.jui@broadcom.com>
Tested-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>