OMAP2xxx IO mapping: mark DSP mappings as being 2420-only

Out of the three major OMAP2 chip types, OMAP2420, OMAP2430, and OMAP3430,
we only map the IVA on OMAP2420.  The memory mapping is not shared between
OMAP2420 and OMAP2430, so it is inappropriate to label those macros as
'24XX'; this patch changes them to '2420'.

Signed-off-by: Paul Walmsley <paul@pwsan.com>