eMMC is supplied by BUCK5 rail. Use the actual regulator instead of
a virtual fixed regulator.

Fixes: 418d1d84 ("arm64: dts: freescale: add initial device tree for TQMa8MPQL with i.MX8MP")
Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>