pinctrl: kinetis: use kernel default init priority

Set the initialization priority for the pinctrl_mcux_init to
CONFIG_KERNEL_INIT_PRIORITY_DEFAULT.

The pinmux nodes depend on pcc, which is currently initializing at a
later stage, using the default priority fixes it.

Found the error with:

$ west build -p -b frdm_k64f samples/basic/blinky \
  -DCONFIG_CHECK_INIT_PRIORITIES=y
...
ERROR: /soc/pinmux@4004d000 PRE_KERNEL_1 0 <
  /soc/pcc@40065000 PRE_KERNEL_1 30
ERROR: /soc/pinmux@4004c000 PRE_KERNEL_1 0 <
  /soc/pcc@40065000 PRE_KERNEL_1 30
ERROR: /soc/pinmux@4004b000 PRE_KERNEL_1 0 <
  /soc/pcc@40065000 PRE_KERNEL_1 30
ERROR: /soc/pinmux@4004a000 PRE_KERNEL_1 0 <
  /soc/pcc@40065000 PRE_KERNEL_1 30
ERROR: /soc/pinmux@40049000 PRE_KERNEL_1 0 <
  /soc/pcc@40065000 PRE_KERNEL_1 30

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