imx: ventana: add fdt fixup to enable UHS-I support on selected boards

imx: ventana: add fdt fixup to enable UHS-I support on selected boards

UHS-I support is available on Ventana boards with micro-SD sockets depending
on the board revision. For backwards compatibility to not break users
who have old bootloaders and newer kernels the device-tree on boards with
microSD disables UHS-I support by default by defining the no-1-8-v property
in the esdhc controller node. For models/revisions that support switchable
1.8V/3.3V I/O which is detectable by the presence of a pull-down on the
SD3_VSELECT pin we remove that property to enable support in the kernel.

Additionally we add SD3_VSELECT to the pinmux for clarity (even though U-Boot
does not currently support UHS-I modes requiring 1.8V I/O).

Signed-off-by: Tim Harvey <tharvey@gateworks.com>