ARM: OMAP: CLKFW: Initial debugfs support for omap clock framework

debugfs can provide the infrastructure to trace the dependencies of
clock tree hierarchy quite visibly. This patch enables to keep track
of clock tree hierarchy and expose their attributes under each clock
directry as below:

	omap:~# tree -d -L 2 /debug/clock/omap_32k_fck/
	/debug/clock/omap_32k_fck/
	|-- gpt10_fck
	|-- gpt11_fck
	|-- gpt1_fck
	|-- per_32k_alwon_fck
	|   |-- gpio2_fck
	|   |-- gpio3_fck
	|   |-- gpio4_fck
	|   |-- gpio5_fck
	|   |-- gpio6_fck
	|   `-- wdt3_fck
	|-- ts_fck
	`-- wkup_32k_fck
	    |-- gpio1_fck
	    `-- wdt2_fck

	14 directories
	omap:~# tree  /debug/clock/omap_32k_fck/gpt10_fck/
	/debug/clock/omap_32k_fck/gpt10_fck/
	|-- flags
	|-- rate
	`-- usecount

	0 directories, 3 files

Although, compared with David Brownell's small patch, this may look
bit overkilling, I expect that this debugfs can deal with other PRCM
complexities at the same time. For example, powerdomain dependencies
can be expressed by using symbolic links of these clocks if
powerdomain supports dubgfs as well.

Signed-off-by: Hiroshi DOYU <Hiroshi.DOYU@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>