sunxi: dts: Fix typoed eMMC check

Commit 03510bf6 ("sunxi: only include alias for eMMC when mmc2
used") protected the eMMC alias in U-Boot's DT stub the with the
associated Kconfig symbol, but was actually using the wrong name.

Fix the name of the symbol to match what's defined in Kconfig and what
the defconfig files actually use.

Fixes: 03510bf6 ("sunxi: only include alias for eMMC when mmc2 used")
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reported-by:  <5kft@5kft.org>
Reviewed-by: Icenowy Zheng <icenowy@aosc.io>