clk/ux500: explicitly include register header

The ux500 clock driver file named u8500_clk.c does not compile
if it loose implicit dependencies to <mach/db8500-regs.h>
(I have no clue what files will include this) which manifests
itself as I'm experimenting with SPARSE_IRQ so make the
dependency explicit.

Cc: Mike Turquette <mturquette@linaro.org>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>