pinctrl: imx: fix scu link errors

Currently PINCTRL_IMX8QM and PINCTRL_IMX8QXP will select PINCTRL_IMX_SCU.
However, PINCTRL_IMX_SCU may not be valid due to it depends on IMX_MBOX.
Then we may meet the following link errors:
ld: drivers/pinctrl/freescale/pinctrl-scu.o: in function `imx_pinctrl_sc_ipc_init':
pinctrl-scu.c:(.text+0x10): undefined reference to `imx_scu_get_handle'
ld: pinctrl-scu.c:(.text+0x10): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol `imx_scu_get_handle'
ld: drivers/pinctrl/freescale/pinctrl-scu.o: in function `imx_pinconf_get_scu':
pinctrl-scu.c:(.text+0xa0): undefined reference to `imx_scu_call_rpc'
ld: pinctrl-scu.c:(.text+0xa0): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol `imx_scu_call_rpc'
ld: drivers/pinctrl/freescale/pinctrl-scu.o: in function `imx_pinconf_set_scu':
pinctrl-scu.c:(.text+0x1b4): undefined reference to `imx_scu_call_rpc'
ld: pinctrl-scu.c:(.text+0x1b4): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol `imx_scu_call_rpc'
ld: drivers/pinctrl/freescale/pinctrl-imx8qxp.o: in function `imx8qxp_pinctrl_probe':
pinctrl-imx8qxp.c:(.text+0x28): undefined reference to `imx_pinctrl_probe'
ld: pinctrl-imx8qxp.c:(.text+0x28): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol `imx_pinctrl_probe'

Rework so that PINCTRL_IMX8QM and PINCTRL_IMX8QXP depends on IMX_SCU
as well in case they're wrongly enabled.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>