riscv: Adjust board_get_usable_ram_top() for 32-bit

When testing QEMU RISC-V 'virt' machine with a 2 GiB memory
configuration, it was discovered gd->ram_top is assigned to
value zero in setup_dest_addr().

While gd->ram_top should not be declared as type `unsigned long`,
which will be updated in a future patch, the current logic in
board_get_usable_ram_top() can be updated to cover both 64-bit
and 32-bit RISC-V.

Signed-off-by: Bin Meng <bin.meng@windriver.com>