arm: allow the debug stuff in CP14 to be disabled at compile time

The upcoming QCA ipq401x support detects the CP14 debug features,
but any attempt to use it causes an undefined instruction error.
It apparently needs a specific TZ image loaded by the early bootloader
(SBL) in order to enable these kinds of features.

So add a new kernel option that explicitly disables this in the
arm code - the debugger works fine without it.