ARM: OMAP: Introduce SoC specific early_init

Introduce them for each omap variant and just make them all call
omap2_init_common_infrastructure for now. Do this for each board-*.c
file except for board-generic and board-omap3beagle as they use
the same machine ID for multiple SoCs.

No functional changes.

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