ARM: OMAP: cleanup struct omap_board_config_kernel

struct omap_board_config_kernel defined in the board files
is always empty and does not bring any added value.
Remove the struct omap_board_config_kernel instances from the board
files.
Also remove the omap_get_nr_config() macro and the omap_get_var_config()
function as both are not used for quite a long time (if ever).

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