common/cmd_source.c: Fix the source command failure under 64-bit platform

Modify the data pointer type from ulong* to u32*.

For arm64 type "ulong" could be 64-bit. Then in line 89 of common/cmd_source.c:
"while (*data++);" data will point to the next 64 bits each time. As the uImage
file generated by mkimage tool keeps the same data format in either 32-bit or 64-bit
platform, the difference would cause failure in 64-bit platform.

Signed-off-by: Gong Qianyu <Qianyu.Gong@freescale.com>