KVM: arm64: selftests: Add write_dbg{b,w}{c,v}r helpers in debug-exceptions

Introduce helpers in the debug-exceptions test to write to
dbg{b,w}{c,v}r registers. Those helpers will be useful for
test cases that will be added to the test in subsequent patches.

No functional change intended.

Signed-off-by: Reiji Watanabe <reijiw@google.com>
Reviewed-by: Ricardo Koller <ricarkol@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221020054202.2119018-3-reijiw@google.com