dt: uartlite: merge platform and of_platform driver bindings

of_platform_driver is getting removed, and a single platform_driver
can now support both devicetree and non-devicetree use cases.  This
patch merges the two driver registrations.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Peter Korsgaard <jacmet@sunsite.dk>