ARM: OMAP4: Fix legacy code clean-up regression

Commit 2a26d31b ("ARM: OMAP2+: Remove unused legacy code for PRM")
removed PRM platform init code that I thought is unused. Turns out omap4
still needs this code, so let's do a partial revert to add it back.

I probably missed this earlier as the comments used to say
"OMAP4+ is DT only now" for !of_have_populated_dt() to exit early and
missed the negative test. Let's not add those lines back as they are
confusing and no longer needed as we only boot in device tree mode.

Without things things can mysterious fail for i2c, for example LM75
I2C temperature sensor can stop working as the PRM interrupts won't work.

Fixes: 2a26d31b ("ARM: OMAP2+: Remove unused legacy code for PRM")
Signed-off-by: Tony Lindgren <tony@atomide.com>