omap: Fix multi.h when only ARCH_OMAP3 and SOC_AM33XX are selected

When only ARCH_OMAP3 (or -2,-4,...) and SOC_AM33XX are selected, multi.h
doesn't set MULTI_OMAP2. In this case, cpu.h will simply define
cpu_is_omap24xx() as 1.

This causes problems for example for omap_hwmod.c:omap_hwmod_init which
checks for cpu_is_omap24xx() first, using the wrong soc_ops for AM33xx.

Fix this by defining MULTI_OMAP2 when using SOC_AM33XX together with
something else.

Signed-off-by: Jan Luebbe <jlu@pengutronix.de>
Signed-off-by: Tony Lindgren <tony@atomide.com>