plat/board.h file is now empty - remove it.

Cc: Jarkko Lavinen <jarkko.lavinen@nokia.com>
Cc: Chris Ball <cjb@laptop.org>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Cc: linux-mmc@vger.kernel.org
Cc: linux-mtd@lists.infradead.org
Signed-off-by: Igor Grinberg <grinberg@compulab.co.il>
Acked-by: Chris Ball <cjb@laptop.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>