24xx clock: clarify clock structure; correct parent clock

Separate the DSP I-clock rate clksel settings from dsp_ick, and use it
to create a new clock, dsp_irate_ick.  Use this new clock as the
parent of dsp_ick and iva2_1_ick.  dsp_ick now simply controls whether
the IPI and ISP clocks are enabled on 2420.

Also, correct the parent clock of the DSP interface clock to be dsp_fck,
not core_ck, per the 2420 TRM figure 5-15 and 2430 TRM figure 4-16.

Fix the OMAP DSP code to use the correct clock name on 2430.  This
avoids the "omapdsp: could not acquire dsp_ick handle" warning that appears
at boot.

While we are here, we also remove some redundant clksel_rate arrays -
iva1_ifck_core_rates[] and iva2_1_ick_core_rates[].

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>