clk: spear: fix ADC clock definition on SPEAr600

There is no SPEAr600 device named "adc". Instead, the description of the
ADC was recently added to the Device Tree, and the device name is
"d820b000.adc", so we should associate the ADC gatable clock to this
device name.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>