ARM: Remove apollon board

This board is the only board that still sticks to OneNAND IPL.
Remove this board, since we have SPL around for a while and
OneNAND is well supported in the SPL framework. The board can
be revived if necessary.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Tom Rini <trini@ti.com>