gpio: npm1300: lower init priority

Lower the priority of the npm1300 gpio driver. The GPIOs are used by
some regulator nodes in the default shield config, but regulators are
initialized before the gpios. Lowering the GPIO priority to sort thigs
out.

Found with the build time priority checking feature:

$ west build -p -b nrf52dk_nrf52832 samples/shields/npm1300_ek \
  -DCONFIG_CHECK_INIT_PRIORITIES=y
...
ERROR: /soc/i2c@40003000/pmic@6b/regulators/LDO2 POST_KERNEL 76 <
  /soc/i2c@40003000/pmic@6b/gpio-controller POST_KERNEL 80
ERROR: /soc/i2c@40003000/pmic@6b/regulators/LDO1 POST_KERNEL 76 <
  /soc/i2c@40003000/pmic@6b/gpio-controller POST_KERNEL 80
ERROR: /soc/i2c@40003000/pmic@6b/regulators/BUCK2 POST_KERNEL 76 <
  /soc/i2c@40003000/pmic@6b/gpio-controller POST_KERNEL 80
ERROR: /soc/i2c@40003000/pmic@6b/regulators POST_KERNEL 75 <
  /soc/i2c@40003000/pmic@6b/gpio-controller POST_KERNEL 80
ERROR: /soc/i2c@40003000/pmic@6b/regulators POST_KERNEL 75 <
  /soc/i2c@40003000/pmic@6b/gpio-controller POST_KERNEL 80
ERROR: /soc/i2c@40003000/pmic@6b/regulators POST_KERNEL 75 <
  /soc/i2c@40003000/pmic@6b/gpio-controller POST_KERNEL 80

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>