riscv: ae350: Fix XIP config boot failure

The booting flow is SPL -> OpenSBI -> U-Boot.
The boot hart may change after OpenSBI and may not always be hart0,
so wrap the related branch instruction with M-MODE.

Current DTB setup for XIP is not valid.
There is no chance for CONFIG_SYS_FDT_BASE, the DTB address used
in XIP mode, to be returned. Fix this.

Fixes: 2e8d2f88 ("riscv: Remove OF_PRIOR_STAGE from RISC-V boards")
Signed-off-by: Rick Chen <rick@andestech.com>
Signed-off-by: Leo Yu-Chi Liang <ycliang@andestech.com>