[ARM] Remove boards already handled by generic board support

Currently, generic board support handles lot of ARM boards such as
bcm2835, imx25, imx31, realview, sun4i, versatile, and vexpress.

This patch removes these board specific code so that we always use
device tree based generic board support for these boards.

After this patch, only Exynos4 and OMAP3 board support will remain
to be merged in generic board support.

Signed-off-by: Anup Patel <anup@brainfault.org>