omap2+: fix build regression

board-generic.c now contains a reference to omap3_timer, but depends
only on ARCH_OMAP2, not on ARCH_OMAP3, which controls that symbol.
omap2_timer seems to be more appropriate anyway, so use that instead.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Tony Lindgren <tony@atomide.com>