serial: imx: Change cast to handle 64-bit resource_size_t

This resolves a warning where resource_size_t is larger than void *:

  drivers/tty/serial/imx.c:1542:6: warning: cast to pointer from integer
  	of different size [-Wint-to-pointer-cast]

Since iomem_base is a void *, casting to unsigned long is safe.

It's unclear to me that this comparison is truly needed, but it's there
on several other drivers as well.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>