configs: am62x_evm_a53_defconfig: Switch to bootstd

configs: am62x_evm_a53_defconfig: Switch to bootstd

Switch to using bootstd. Note with this change, we will stop using
distro_bootcmd and instead depend entirely on bootflow method of
starting the system up.

Suggested-by: Tom Rini <[email protected]>
Suggested-by: Simon Glass <[email protected]>
Reviewed-by: Tom Rini <[email protected]>
Tested-by: Mattijs Korpershoek <[email protected]>
Signed-off-by: Nishanth Menon <[email protected]>
Reviewed-by: Tom Rini <[email protected]>
Reviewed-by: Mattijs Korpershoek <[email protected]>