ARM: OMAP2+: Fix booting for am335x after moving to simple-pm-bus

We now depend on SIMPLE_PM_BUS for probing devices. While we have it
selected in omap2plus_defconfig, custom configs can fail if it's missing.

As SIMPLE_PM_BUS depends on OF and PM, we must now select PM in Kconfig.
We have already OF selected by ARCH_MULTIPLATFORM.

Let's also drop the earlier PM dependency entries as suggested by
Geert Uytterhoeven <geert@linux-m68k.org>.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reported-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Fixes: 5a230524

 ("ARM: dts: Use simple-pm-bus for genpd for am3 l4_wkup")
Signed-off-by: Tony Lindgren <tony@atomide.com>