Use platform_device_register_simple() rather than a static
struct, so we create and register the PMU device on-the-fly.

Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Rabin Vincent <rabin.vincent@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>