fdt: make fdt_get_base_address() return OF_BAD_ADDR when "reg" not found

fdt: make fdt_get_base_address() return OF_BAD_ADDR when "reg" not found

Currently, fdt_get_base_address() returns 0 if the "reg" property is
missing. Since 0 is a valid value, it is not suitable for the error
handling. Return OF_BAD_ADDR instead.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>