ARM: pxa: Fix OneNAND window access on VPAC270

Access the OneNAND 1KiB window on the VPAC270 as an SRAM instead of accessing
it as a burst-RAM. This fixes a problem where the board failed to reboot
sometimes as the CPU couldn't start executing from the OneNAND 1KiB window.

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