arm: iop: specify PMUs are for XScale CPUs

Now that we can specify which PMU variant we're likely to deal with, do
so in the iop 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: Linus Walleij <linus.walleij@linaro.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Olof Johansson <olof@lixom.net>
Cc: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Olof Johansson <olof@lixom.net>