xen/arm: Implement a dummy debug monitor for ARM32

xen/arm: Implement a dummy debug monitor for ARM32

XSA-93 (commit 0b18220 "xen/arm: Don't let guess access to Debug and Performance
Monitors registers") disable Debug Registers access.

When CONFIG_PERF_EVENTS is enabled in the Linux Kernel, it will try to
initialize the debug monitors. If an error occured Linux won't use this
feature.

The implementation made Xen expose a minimal set of registers which let think
the guest (i.e.) thinks HW debug won't work.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
[ ijc -- s/DBGCR/DBGBCR/ to use correct register name ]
Acked-by: Ian Campbell <ian.campbell@citrix.com>