pinctrl: imx-scu: Depend on IMX_SCU

pinctrl: imx-scu: Depend on IMX_SCU

Otherwise building fails with only PINCTRL_IMX_SCU selected:

    aarch64-linux-gnu-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'
    aarch64-linux-gnu-ld: pinctrl-scu.c:(.text+0x10): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol `imx_scu_get_handle'
    aarch64-linux-gnu-ld: drivers/pinctrl/freescale/pinctrl-scu.o: in function `imx_pinconf_get_scu':
    pinctrl-scu.c:(.text+0x64): undefined reference to `imx_scu_call_rpc'
    aarch64-linux-gnu-ld: pinctrl-scu.c:(.text+0x64): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol `imx_scu_call_rpc'
    aarch64-linux-gnu-ld: drivers/pinctrl/freescale/pinctrl-scu.o: in function `imx_pinconf_set_scu':
    pinctrl-scu.c:(.text+0x104): undefined reference to `imx_scu_call_rpc'
    aarch64-linux-gnu-ld: pinctrl-scu.c:(.text+0x104): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol `imx_scu_call_rpc'
    make: *** [Makefile:1038: vmlinux] Error 1

Signed-off-by: Guido Günther <agx@sigxcpu.or>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>