soc: ti: pruss: support CORECLK_MUX and IEPCLK_MUX

The IEPCLK_MUX is present on all SoCs whereas the CORECLK_MUX is present
only on AM65x SoCs and J721E. Add support for both these CLK muxes.

This allows the clock rates and clock parents for these to be controlled
through DT leveraging the clk infrastructure for configuring the default
parents and rates.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Grzegorz Jaszczyk <grzegorz.jaszczyk@linaro.org>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>