riscv: image: Use the first DRAM bank for bootm_low

riscv: image: Use the first DRAM bank for bootm_low

bootm_low is used as a base address is used to allocate space for the
FDT blob, initrd, cmdline, etc. when booting Linux. Set the default
value for RISC-V to the start of the first DRAM bank, so platforms can
get their DRAM layout from the device tree, and do not need to define
CONFIG_SYS_SDRAM_BASE.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>