ARM: OMAP: omap3evm: cleanup revision bits

The omap3evm has its revision information bits inside the plat/board.h
file. Those bits are not used anywhere in the upstream tree besides the
board-omap3evm.c file.
Move the OMAP3EVM_BOARD_GEN_* bits to the board file and remove the
get_omap3_evm_rev() function declaration.

Signed-off-by: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: Tony Lindgren <tony@atomide.com>