ARM: dts: am437x-gp-evm: Rename fixed regulator vmmcsd_fixed to evm_v3_3d

The same regulator is used by other chips on the board. The power path is:
VBAT -> TPS63031 - Enable signal is V1_8D regulator -> V3_3D.
V3_3D is used by SD slot and tlv320aic3106 codec as well.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>