omap2/3: Fix DEBUG_LL for omap zoom2/3

Zoom2 and 3 have UARTs only on the external debug board.
GPMC needs to be mapped early to use it for DEBUG_LL.

Additionally, 0xfb000000 overlaps with other areas, so
use 0xfa400000 for the virtual address instead.

Note that with the pending serial.c patches you need to
set console=ttyS0,115200n8 as it will be the only UART
mapped. To use DEBUG_LL, you need to pass also earlyprintk
in cmdline.

Cc: Allen Pais <allen.pais@ti.com>
Acked-by: Vikram Pandita <vikram.pandita@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>