drm/sun4i: add COMMON_CLK dependency

The sun4i drm driver uses the clk-provider interfaces, which are not available
when CONFIG_COMMON_CLK is disabled:

drivers/gpu/drm/sun4i/sun4i_dotclock.c:19:16: error: field 'hw' has incomplete type
  struct clk_hw hw;
In file included from ../include/asm-generic/bug.h:13:0,
                 from ../arch/arm/include/asm/bug.h:59,
                 from ../include/linux/bug.h:4,
                 from ../include/linux/io.h:23,
                 from ../include/linux/clk-provider.h:14,
                 from ../drivers/gpu/drm/sun4i/sun4i_dotclock.c:13:
drivers/gpu/drm/sun4i/sun4i_dotclock.c: In function 'hw_to_dclk':
include/linux/kernel.h:831:48: error: initialization from incompatible pointer type [-Werror=incompatible-pointer-types]
...

This adds a Kconfig dependency to prevent the driver from being enabled
in this case.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 9026e0d1 ("drm: Add Allwinner A10 Display Engine support")
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>