mfd: ab8500-core: Rename clock device and compatible

We didn't have proper device tree bindings for this clock,
I standardized it to use the exact chipname so let's rename
it "ab8500-clk" and rectify the device tree compatible string
to "stericsson,ab8500-clk".

Cc: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>