riscv: Set SYS_BOOTM_LEN explicitly to 0x800000

riscv: Set SYS_BOOTM_LEN explicitly to 0x800000

For all riscv defconfigs that use the current default value.

This is done in provision of changing the default value to the most
common used value of 0x4000000.

Signed-off-by: Martin Herren <sputnik@on-the-web.ch>
Acked-by: Michal Simek <michal.simek@amd.com> # xilinx_mbv
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>