ARM: dts: stm32: Fix AV96 eMMC pinmux

ARM: dts: stm32: Fix AV96 eMMC pinmux

Commit 500327e2ea7 ("ARM: dts: stm32mp1: DT alignment with Linux kernel v5.8-rc1")
renamed sdmmc2_d47_pins_b phandle to sdmmc2_d47_pins_c, but without updating
the AV96 DT which uses that phandle. Linux missed similar update as well and
it was only added in commit 1ad6e36ec266 ("ARM: dts: stm32: Fix sdmmc2 pins
on AV96") .

Update the AV96 DT pinmux phandle, otherwise eMMC 8bit mode does not work
and access to eMMC takes a very long time to fall back to 4bit mode.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Patrick Delaunay <patrick.delaunay@foss.st.com>
Cc: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>