riscv: qemu: support booting Linux

Support booting Linux (as payload of BBL) from FIT images. For this, the
default CONFIG_SYS_BOOTM_LEN is increased to 16 MB, and the environment
variables fdt_high and initrd_high are set to mark the device tree and
initrd as in-place.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>