riscv: sifive: unmatched: Adjust for big ramdisk image

Move kernel_comp_addr_r to an address that comes before the ramdisk
image, since the decompressed kernel size is known to us. This way
we can allow big ramdisk image to be loaded.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>