Move pca953x support from i2c.mk to other.mk, where other
GPIO drivers are located. Remove duplicate pcf857x support
from i2c.mk.

Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
[juhosg: update subject, and reformat commit message]
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

SVN-Revision: 38185