soc: lpc11u6x: fix pinmux initialization priority

The commit aac9e2c5
("device: Revise how initialization status is being handled") highlights
an initialization priority issue between the clock and pinmux device.
Since this commit Zephyr is not booting anymore on LPC11U6x MCUs. The
clock driver gets a NULL pointer when calling device_get_binding() to
retrieve the pinmux device. It is because the pinmux device is not
initialized yet due to a lesser priority.

This patch fixes this issue by ensuring that Zephyr initializes the
pinmux device before the clock device.

Signed-off-by: Simon Guinot <simon.guinot@seagate.com>