clk: Include of.h in clock-provider.h

CLK_OF_DECLARE relies on OF_DECLARE_1 that is defined in of.h. Fixes build
errors when one use CLK_OF_DECLARE but doesn't include of.h

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>