<linux/clk.h> should be included to get the base API prototypes.

This fixes the following sparse warnings:

  arch/arm/common/clkdev.c:65:12:
    warning: symbol 'clk_get_sys' was not declared. Should it be static?

  arch/arm/common/clkdev.c:79:12:
    warning: symbol 'clk_get' was not declared. Should it be static?

  arch/arm/common/clkdev.c:87:6:
    warning: symbol 'clk_put' was not declared. Should it be static?
Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>