omap clean-up for v4.2

Clean-up for omaps for v4.2 merge window:

Drop more omap3 legacy board-*.c files for v4.2. This time we're dropping the board files for beagle, overo and cm-t35.

The reason for dropping these now rather than later is that now we can simply revert the patches in case of unexpected issues as we are not dropping any platform data at this point.

This leaves us with only the following board-*.c files:

BOARD                   REASON NEEDED
board-ldp.c             Still used in RMKs boot test system
board-omap3logic.c      Getting a .dts file for v4.2
board-omap3pandora.c    Got a .dts file for v4.1
board-rx51-*.c          Needs /proc/atags support for user space

I'm hoping we can drop the remaining board-*.c for v4.3 merge window, then keep the platform data around for one more merge window in case of reverting needed, then drop the platform data too for v4.4 if no issues needing reverting are found.