pinctrl: imx: Split MMIO accessors into pinctrl-imx-mmio.c

Split MMIO accessors into pinctrl-imx-mmio.c and build this
file only if Kconfig symbol PINCTRL_IMX_MMIO is selected.
Select PINCTRL_IMX_MMIO Kconfig symbol for all but pinctrl-imx8.c
driver, which does not use the MMIO accessors. This reduces the
amount of code compiled on platforms which do not use the code.

No functional change.

Signed-off-by: Marek Vasut <marex@denx.de>
diff --git a/drivers/pinctrl/nxp/Makefile b/drivers/pinctrl/nxp/Makefile
index c82275a..44e37c6 100644
--- a/drivers/pinctrl/nxp/Makefile
+++ b/drivers/pinctrl/nxp/Makefile
@@ -1,4 +1,5 @@
 obj-$(CONFIG_PINCTRL_IMX)		+= pinctrl-imx.o
+obj-$(CONFIG_PINCTRL_IMX_MMIO)		+= pinctrl-imx-mmio.o
 obj-$(CONFIG_PINCTRL_IMX5)		+= pinctrl-imx5.o
 obj-$(CONFIG_PINCTRL_IMX6)		+= pinctrl-imx6.o
 obj-$(CONFIG_PINCTRL_IMX7)		+= pinctrl-imx7.o