When described in DT, PMUs are given specific compatible strings
(e.g. "arm,cortex-a15-pmu") which makes it very easy to reorganise the
way individual PMUs are handled (i.e. we can easily split them into
separate drivers). The same is not true of PMUs described in board
files, which are all use the platform_device_id "arm-pmu" and must all
be handled by the same driver.

To enable splitting the ARMv6, ARMv7, and XScale PMU drivers we need
board files to identify which variant they provide. As a first step,
this patch adds new platform_device_id values: "armv6-pmu", "armv7-pmu,
and "xscale-pmu".

Once board files are moved over and all existing uses of "arm-pmu" are
gone, we can split the existing driver apart.
Acked-by: Will Deacon <will.deacon@arm.com>
Tested-by: Christopher Covington <cov@codeaurora.org>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>