staging:iio:adc:spear_adc use info_mask_shared_by_all for samp freq

Using the core support makes this element available to in kernel users as
well as to userspace under exactly the same interface as before.  The
intent is to move all sampling frequency control to this approach
throughout IIO.

Drop unused clk_high and clk_low whilst we are at it.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Acked-by: Stefan Roese <sr@denx.de>
Acked-by: Hartmut Knaack <knaack.h@gmx.de>