arch-arm: Rename SelfDebug member variables

arch-arm: Rename SelfDebug member variables

* enableFlag -> mde
The "enableFlag" variable, enabling the Breakpoint, Watchpoint, Vector
Catch exceptions is actually the cached version of MDSCR_EL1.MDE. The
"enableFlag" name looks too general as it's not covering the Software
Step exception case.

* bKDE -> kde
* bSDD -> sdd

The b prefix was likely referring to "breakpoint". However these bitfields
are actually used by watchpoints as well.

Change-Id: I48b762b32b2d763f4c4ceb7dcc28968cfb470fc1
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32775
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Richard Cooper <richard.cooper@arm.com>