spi/stmp: remove obsolete driver

This driver is obsolete and can't even be built anymore since the
platform it depends has been removed.

The STMP series is completely covered by the MXS platform these days, so
spi-mxs can be used instead.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>