Convert OMAPs 32kHz clocksource implementation to use the generic MMIO

Convert OMAPs 32kHz clocksource implementation to use the generic MMIO
clocksource support.  This achieves several things:

1. It means we get rid of all these helper functions which frankly should
   never have been necessary.
2. It means omap_readl() inside these helper functions does not appear in
   ftrace output.

Another plus is that we avoid the overhead of calculating the address to
read each time, but a minus is that we use readl() which has a barrier.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
[tony@atomide.com: updated to use ioremap]
Signed-off-by: Tony Lindgren <tony@atomide.com>