board: ae350: Support autoboot from RAM

Add boot command "bootcmd_ram" to support autoboot from RAM.

This feature could be useful at the very initial state of chip design
when there is only a minimal set of peripheral. (e.g. without mmc and mac ..etc)

The kernel image is default to be loaded at 0x2000000 via debug port,
and the following script serves as an example:

spl()
{
	cmd="riscv64-linux-gdb -q \
	-ex \"target remote $host:$port\" \
	-ex \"load\" \
	-ex \"thread apply all set \\\$pc=&_start\" \
	-ex \"thread apply all set \\\$a0=\\\$mhartid\" \
	-ex \"thread apply all set \\\$a1=<dtb address>\" \
	-ex \"restore u-boot.itb binary 0x200000\" \
	-ex \"restore Image binary 0x2000000\" \
	-ex \"c\" \
	spl/u-boot-spl
	"

	echo $cmd
	eval $cmd
}

The address where the kernel is loaded can be altered by
changing the value of KERNEL_IMAGE_ADDR.

Signed-off-by: Leo Yu-Chi Liang <ycliang@andestech.com>
Reviewed-by: Rick Chen <rick@andestech.com>