clk: ux500: Ensure the FMSC clock is obtainable

The FMSC clock is traditionally used for NAND flash devices when
used on the ux500 series platforms. This patch makes it searchable
during a clock-name search.

Acked-by: Mike Turquette <mturquette@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>