uboot-imx

openrisc: Fix a few type cast related warnings

Use size_t type for positive offsets instead of the loff_t type. The
later is defined as long long, which is larger than the pointer type
on OpenRISC architecture and therefore the following warning was
generated:

"warning: cast to pointer from integer of different size"

Signed-off-by: Vasili Galka <vvv444@gmail.com>