arm: nvic: kill _NvicSwInterruptTrigger

_NvicSwInterruptTrigger is only utilized by a testcase for irq handling
on ARM-V7M.  Just put the code into the testcase so we dont need to
support an additional interface.

Change-Id: I763c63c32a7a52918250458351d08b8fa54069dd
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>