riscv: ax25: add SPL support

The U-Boot SPL will boot in M mode and load the FIT image which
include OpenSBI and U-Boot proper images. After loading progress,
it will jump to OpenSBI first and then U-Boot proper which will
run in S mode.

Also remove V5L2_CACHE due to U-Boot SPL code size consideration.
Without this concern, it can be enable manually for performance.

Signed-off-by: Rick Chen <rick@andestech.com>
Cc: KC Lin <kclin@andestech.com>
Cc: Alan Kao <alankao@andestech.com>