ARM: OMAP2+: Fix am4 only build after genpd changes

With commit df6c2ec8 ("ARM: OMAP2+: Drop legacy remaining legacy
platform data for am4") we moved am4 to boot with simple-pm-bus using
genpd and devicetree based data. But I forgot to test am4 only build
that still has few references to the old platform data left, and cause
undefined reference errors with omap_hwmod_set_postsetup_state and
omap_hwmod_for_each. We can just drop the related calls for am4 now,
and also drop the references to unused struct wkup_m3_platform_data.

Reported-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>