arm: pxa: specify PMUs are for XScale CPUs

Now that we can specify which PMU variant we're likely to deal with, do
so in the pxa board code. This will allow us to split the ARMv6, ARMv7,
and XScale PMU drivers.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Daniel Mack <daniel@zonque.org>
Cc: Haojian Zhuang <haojian.zhuang@gmail.com>
Cc: Olof Johansson <olof@lixom.net>
Signed-off-by: Olof Johansson <olof@lixom.net>