[ARM] Handle +t2dsp feature as an ArchExtKind in ARMTargetParser.def

Currently, the availability of DSP instructions (ACLE 6.4.7) is handled in a
hand-rolled tricky condition block in tools/clang/lib/Basic/Targets.cpp, with
a FIXME: attached.

This patch changes the handling of +t2dsp to be in line with other
architecture extensions.

Following review comments, also updating the description of FeatureDSPThumb2
in ARM.td.

Differential Revision: http://reviews.llvm.org/D12937

llvm-svn: 248152