regulator: Fix compile break due to missing arguments to regulator_register

The commit 2c043bcb ("regulator: pass additional of_node to
regulator_register()") caused a compile break because it missed
updating the regulator_register() call in gpio-regulator.c with
the additional parameter (NULL).

The compile break as reported by Stephen Rothwell with the
x86_64 allmodconfig looked like this

drivers/regulator/gpio-regulator.c: In function 'gpio_regulator_probe':
drivers/regulator/gpio-regulator.c:287:8: error: too few arguments to function 'regulator_register'
include/linux/regulator/driver.h:215:23: note: declared here

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Rajendra Nayak <rnayak@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>