TEMP: soc: ti: pruss: Extend IEPCLK_MUX support to non-K3 SoCs

The commit 1b7637e5 ("TEMP: soc: ti: pruss: support CORECLK_MUX
and IEPCLK_MUX") has added limited support for the internal PRU-ICSS
mux clocks IEPCLK_MUX and CORECLK_MUX only on K3 AM65x and J721E
SoCs.

The IEPCLK_MUX is present on the other SoC families supporting PRUSS
like the OMAP-architecture SoCs AM335x, AM437x, AM57xx and the
Keystone 2 66AK2G SoCs. Enhance the PRUSS platform driver to extend
the IEPCLK_MUX support on all these SoCs.

While at this, a small typo is also fixed that used the wrong clk
variable during the IEPCLK_MUX clock registration.

Signed-off-by: Suman Anna <s-anna@ti.com>