board: ti: am64x: Switch to standard boot flow

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.

Drop header files that are no longer needed in am64x_evm.h.
k3_dfu.h is available via k3_dfu.env in am64x.env.

Drop unused macro CFG_SYS_SDRAM_BASE1.

Signed-off-by: Roger Quadros <rogerq@kernel.org>
Reviewed-by: Nishanth Menon <nm@ti.com>