ARM: OMAP2+: Improve handling of ti-sysc related sysc_fields

We can currently get a cryptic warning sysc_fields (ptrval) != (ptrval)
if the legacy platform data has no sysc_fields defined while the newer
dts data has them. This warning appears only when booting still with
legacy "ti,hwmods" custom property set. This can happen at least with
DSS related modules where we may not have sysc_fields defined in the
in the legacy data.

Let's not error out on missing legacy data sysc_fields, and show a more
descriptive warning for other cases.

Signed-off-by: Tony Lindgren <tony@atomide.com>