riscv: sifive: unleashed: Set kernel_comp_addr_r for compressed kernel

Set kernel_comp_addr_r and kernel_comp_size for compressed kernel.
Adjust existing addresses for ramdisk, so that kernel_comp_addr_r
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.

Update unleashed.rst to remove the manual environment configuration
for compressed kernel boot.

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