asm-generic: Add default clkdev.h

Ease the deployment of clkdev by providing a default asm/clkdev.h for
use if the arch does not have an include/asm/clkdev.h.

Due to limitations in Kbuild we manually add clkdev.h to all
architectures that don't have one rather than having the header appear
by default.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Reviewed-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>