clk: ti: Improve clksel clock bit parsing for reg property

Because of legacy reasons, the TI clksel composite clocks can have
overlapping reg properties, and use a custom ti,bit-shift property.

For the clksel clocks we can start using of the standard reg property
instead of the custom ti,bit-shift property.

To do this, let's add a ti_clk_get_legacy_bit_shift() helper, and make
ti_clk_get_reg_addr() populate the clock bit offset.

This makes it possible to update the devicetree files to use the reg
property one clock at a time.

Acked-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>