uartlite: Fix build on sparc.

We can get this driver enabled via MFD_TIMBERDALE which only
requires GPIO to be on.

But the of_address_to_resource() function is only present on
powerpc and microblaze, so we have to conditionalize the
CONFIG_OF probing bits on that.

Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>