drivers: pinctrl: update lpc pinctrl driver for lpc11u6x

Update pin control driver for lpc11u6x. This SOC does not have a HAL,
so fsl_clock is not available. It also lacks a slew-rate field in the
IOCON register, so this property must be optional.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>