mmc: omap_hsmmc: Cleanup use of cpu_is_* for debounce_clock

There really does not seem to be a need to use cpu_is_* check for getting
the debounce clock as clkdev is perfectly capable of handling situations
when certain clocks are only available on select platforms.

Also get rid of the 'got_dbclk' flag and instead use the dbclk clock
pointer to know if a valid debounce clock exists for the platform.

Signed-off-by: Rajendra Nayak <rnayak@ti.com>
Signed-off-by: Venkatraman S <svenkatr@ti.com>
Signed-off-by: Chris Ball <cjb@laptop.org>