riscv: ax25: bypass malloc when spl fit boots from ram

When fit image boots from ram, the payload will
be prepared in the address of SPL_LOAD_FIT_ADDRESS.
In spl fit generic flow, it will malloc another
memory address and copy whole fit image to this
malloc address.  But it is un-necessary for booting
from RAM.

This patch improves this flow by declare the
board_spl_fit_buffer_addr() to replace the original one.
The larger image size (eq: Kernel Image 10~20MB), it
can save more booting time.

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