mfd: ab8500: Fix ab8500-clk typo

Commit f4d41ad8 ("mfd: ab8500: Example using new OF_MFD_CELL MACRO")
has a typo error renaming "ab8500-clk" to "abx500-clk"
with the result att ALSA SoC audio broke as the clock
driver was not probing anymore. Fixed it up.

Fixes: f4d41ad8 ("mfd: ab8500: Example using new OF_MFD_CELL MACRO")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Stephan Gerhold <stephan@gerhold.net>
Signed-off-by: Lee Jones <lee.jones@linaro.org>