configs: introduce configs for the am62a

Introduce the minimum configs, only SD-MMC and UART boot related
settings, to serve as a good starting point for the am62a as we add more
functionality.

Signed-off-by: Bryan Brattlof <bb@ti.com>
[trini: Disable CONFIG_NET as it's not used, in both platforms]
Signed-off-by: Tom Rini <trini@konsulko.com>