ARM: davinci: move davinci_clk_init() to init_time

This moves the call of davinci_clk_init() from map_io to init_time for all
boards.

This is the proper place to init clocks. This is also done in preparation
for moving to the common clock framework.

dm646x is a special case because we need to handle different ref_clk rates
depending on which board is being used. The clock init in this case is
modified to set the rate before registering the clocks instead of using
davinci_set_refclk_rate() to recalculate the entire clock tree after all
of the clocks are registered.

Also, the cpu_clks field is removed from struct davinci_soc_info since it
is no longer needed.

Signed-off-by: David Lechner <david@lechnology.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>