riscv: Add mem kernel parameter support

The memblock_enforce_memory_limit() could change the memblock
range, so move the dram_end assignment after it in bootmem_init(),
then support mem= cmdline.

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>