Add a generic mmio clocksource, covering both 32-bit and 16-bit register
access sizes, for up or down counters.  This can be used to easily
create clocksources for simple counter-based implementations.

Cc: Alessandro Rubini <rubini@unipv.it>
Cc: Colin Cross <ccross@android.com>
Cc: Eric Miao <eric.y.miao@gmail.com>
Cc: Erik Gilling <konkers@android.com>
Acked-by: N"Hans J. Koch" <hjk@hansjkoch.de>
Cc: Imre Kaloz <kaloz@openwrt.org>
Cc: Krzysztof Halasa <khc@pm.waw.pl>
Cc: Kukjin Kim <kgene.kim@samsung.com>
Cc: Lennert Buytenhek <kernel@wantstofly.org>
Cc: Linus Walleij <linus.walleij@stericsson.com>
Cc: linux-omap@vger.kernel.org
Acked-by: NNicolas Pitre <nico@fluxnic.net>
Cc: Olof Johansson <olof@lixom.net>
Tested-by: NSascha Hauer <s.hauer@pengutronix.de>
Reviewed-by: NThomas Gleixner <tglx@linutronix.de>
Tested-by: NTony Lindgren <tony@atomide.com>
Reviewed-by: NViresh Kumar <viresh.kumar@st.com>
Cc: Wan ZongShun <mcuos.com@gmail.com>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>