omap2/3/4: Fix multi.h when omap3 and omap4 are selected without omap2

Otherwise we'll get an error about get_irqnr_and_base being defined
twice. Add an entry for omap4, and use ARCH_OMAP3 for omap3 instead of
ARCH_OMAP3430.

Also fix the check for omap1 to use ARCH_OMAP2PLUS to avoid having to
add ARCH_OMAP4 separately there.

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