Add platform device and select the correct implementation automatically
depending on wether the old samsung_clock or the common clock framework
is enabled.

This is only done for machines already using the old dclk implementation,
as everybody else should move to use dt anyway.

The machine-specific settings for the external clocks will have to be set
by somebody with knowledge about the specific hardware.
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
[pebolle@tiscali.nl: pointed out typo and fixed]
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>