target/arm: implement armv8 PMUSERENR (user-mode enable bits)

In armv8, this register implements more than a single bit, with
fine-grained enables for read access to event counters, cycles
counters, and write access to the software increment. This change
implements those checks using custom access functions for the relevant
registers.

Signed-off-by: Andrew Baumann <Andrew.Baumann@microsoft.com>
Message-id: 20170228215801.10472-2-Andrew.Baumann@microsoft.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
[PMM: move a couple of access functions to be only compiled
 ifndef CONFIG_USER_ONLY to avoid compiler warnings]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
