dts: arm: Rename ARM NVIC compatibility

Reworking the ARM NVIC binding and cleaning it up.  Towards this
introduce a new compatibility for this new binding.  So we rename
arm,armv{6,7}-nvic to arm,v{6,7}-nvic (for new binding).  We also just
use the bit more generic arm,nvic in device tree files.

Change-Id: I5a2c45313ed94619d9268f2c035dacbc8acded29
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>