ARM: dts: aspeed: ast2600evb: Add sdhci node and gpio regulator for A2 evb.

ARM: dts: aspeed: ast2600evb: Add sdhci node and gpio regulator for A2 evb.

AST2600 A2 (or newer) EVB has gpio regulators for toggling signal voltage
between 3.3v and 1.8v, the patch adds sdhci node and gpio regulator in
the dts file and adds comment for describing the reference design.

Signed-off-by: Steven Lee <steven_lee@aspeedtech.com>
Acked-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20210524073308.9328-2-steven_lee@aspeedtech.com
Signed-off-by: Joel Stanley <joel@jms.id.au>