lantiq: Add a rewritten version of the SPI driver

lantiq: Add a rewritten version of the SPI driver

The new driver provides a few improvements over the old one:
- Separate compatible strings per SoC type (this allows removing some
  hardcoded of_device_is_compatible() checks)
- It does not rely upon spi-bitbang anymore
- chip-selects are numbered as in the datasheet (= starting at 1 instead
  of 0)

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@48292 3c298f89-4303-0410-b956-a3cf2f4a3e73