ARM: OMAP2+: timer: add support for fetching fck handle from DT

The mux clock handle shall be provided via "fck" DT handle. This avoids
the need to lookup the main clock via hwmod core, which will not work
with the clkctrl clock support anymore; the main clock is not going to
be a mux.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>