at91: upgrade to kernel 3.8.7 and use devicetree

at91: upgrade to kernel 3.8.7 and use devicetree

This upgrades the AT91 target to 3.8.7, and migrates to device tree.
This allows a single kernel to be built for most at91 variants which
simplifies things quite a bit. The immediate result is that all
subtargets are nuked, and any boards without dts files are no longer
supported, though the target now includes more boards than before. The
adc driver was also nuked as 3.8.7 includes a new one under the IIO
subsystem.

Signed-off-by: Adam Porter <porter.adam@gmail.com>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@36452 3c298f89-4303-0410-b956-a3cf2f4a3e73