drivers: pinctrl: Kconfig.imx: enable PINCTRL_IMX by default

Currently, when using the `pinctrl_imx.c` driver one has
to manually enable `CONFIG_PINCTRL_IMX` even if the pinctrl
node using the driver is enabled. This is redundant and prone
to error as one would expect the driver to be enabled when
the node is also enabled.

This commit fixes the issue by enabling `CONFIG_PINCTRL_IMX`
by default and adding a new dependency on the state of the
pinctrl node (i.e: node needs to be enabled for the configuration
to also be set to `y`).

Signed-off-by: Laurentiu Mihalcea <laurentiu.mihalcea@nxp.com>