mfd: twl-core: Add API to query the HFCLK rate

CFG_BOOT register's HFCLK_FREQ field hold information about the used HFCLK
frequency.
Add possibility for users to get the configured rate based on this
register.
This register was configured during boot, without it the chip would not
operate correctly, so we can trust on this information.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>