omap1: Use gen_nand

Since omapnand driver never find its way into mainline, switch to gen_nand instead.
Following patch is compile tested only, but it is based on code I wrote for
NetStar board and runtime tested it there.

Signed-off-by: Ladislav Michl <ladis@linux-mips.org>
Cc: Imre Deak <imre.deak@nokia.com>
Cc: Brian Swetland <swetland@google.com>
Cc: Kevin Hilman <kjh@hilman.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>