clk: ti: dflt: move support for default gate clock to clock driver

clk: ti: dflt: move support for default gate clock to clock driver

With the legacy support gone, OMAP2+ default gate clock can be moved
under clock driver. Create a new file for the purpose, and clean-up
the header exports a bit as some clock APIs are no longer needed
outside clock driver itself.

Signed-off-by: Tero Kristo <t-kristo@ti.com>