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.

Also, few patches to clean-up the usage of Linux generic
macros:

- Use define for console loglevel

- Use const for struct irq_domain_ops

- Use PTR_ERR_OR_ZERO instead of PTR_RET
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