riscv: Use optimized version of fdtdec_get_addr_size_no_parent

fdtdec_get_addr_size_no_parent is not an optimized version if parent
node is already available with the caller.

Use fdtdec_get_addr_size_auto_parent to read the "reg" property

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Bin Meng <bin.meng@windriver.com>