PUE requires PKE to mean something, as do pull values with PUE, so do not
compell users to explicitly use PKE and PUE everywhere. This is also what is
done on Linux and what has already been done for i.MX51.

By the way, remove some unused pad control definitions.

There is no change of behavior.

Note that SPI_PAD_CTRL was defined by several boards with a pull value, but
without PKE or PUE, which means that no pull was actually enabled in the pad.
This might be a bug in those boards, but this patch does not change the
behavior, so it just removes the meaningless pull value from those definitions.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>