i2c: pxa: Use i2c-core to get bus number now

The commit: "i2c-core: dt: Pick i2c bus number from i2c alias if
present" adds support for automatically picking the bus number based
on the alias ID.  Remove the now unnecessary code from i2c-pxa that
did the same thing.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>