[POWERPC] perfmon2: make pm_interval register read/write

The pm_interval register in the Cell PMU is read/write, but was implemented in
the kernel as write-only. Previously, the written value was saved in a "shadow"
copy so calls to cbe_read_pm() could return the value.

Perfmon2 needs to be able to read the current values of pm_interval, so change
cbe_read_pm() to read the actual register instead of the "shadow" copy. There
is currently no code in the kernel that tries to read the pm_interval register
with cbe_read_pm() (expecting to receive the "shadow" value), so this should
not break any existing code.

Signed-off-by: Kevin Corry <kevcorry@us.ibm.com>
Signed-off-by: Carl Love <carll@us.ibm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>