aspeed-ast2600-evb.dts was modified for supporting A2 evb.

Since A1/A0 evbs don't have GPIO regulators and SD clock frequency
(SCU210) is different to A2 as well. Adding a new dts that removes new
nodes created in aspeed-ast2600-evb.dts is necessary.
Signed-off-by: Steven Lee <steven_lee@aspeedtech.com>
Acked-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20210524073308.9328-4-steven_lee@aspeedtech.comSigned-off-by: Joel Stanley <joel@jms.id.au>