configs: am64x_evm_a53: Enable MMC UHS modes

configs: am64x_evm_a53: Enable MMC UHS modes

Enable UHS modes for SD by enabling configs for voltage regulator
drivers, IO voltage switching, and configs to support UHS modes.

The am64x SoC has an internal LDO which does voltage switching,
but the MMC_IO_VOLTAGE config is still required to be able to
switch voltage for SD.

While we are here, am64x HS400 mode has been descoped as per
datasheet [0] even though we still initialize to HS200, clean
this up by switching to MMC_HS200_SUPPORT config options.

[0] https://www.ti.com/lit/gpn/am6442

Signed-off-by: Judith Mendez <jm@ti.com>