pinctrl: pinctrl-imx: improve the code comments of PIN_FUNC_ID

The current code comments of PIN_FUNC_ID actually is not true for
SHARE_MUX_CONF_REG case which should be a 4 u32 PIN_FUNC_ID.
Fix the comments and re-org it a bit for better extendibility
as we may add a different size for SCU based PIN_FUNC_ID later.

Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Stefan Agner <stefan@agner.ch>
Cc: Pengutronix Kernel Team <kernel@pengutronix.de>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>