MPC85xx: TQM85xx: fix flash protection for boot loader

As the reset vector is located at 0xfffffffc, all flash sectors from the
beginning of the U-Boot binary to 0xffffffff must be protected. On the
TQM8548-AG having small sectors at the end of the flash it happened that
the last two sector were not protected and an "erase all" left an
un-bootable system behind:

Bank # 2: CFI conformant FLASH (32 x 16)  Size: 32 MB in 270 Sectors
  AMD Standard command set, Manufacturer ID: 0xEC, Device ID: 0x257E
  Erase timeout: 8192 ms, write timeout: 1 ms

  FFFA0000 E RO   FFFC0000   RO   FFFE0000   RO   FFFE4000   RO   FFFE8000   RO
  FFFEC000   RO   FFFF0000   RO   FFFF4000   RO   FFFF8000 E      FFFFC000

The same bug seems to be in drivers/mtd/cfi_flash.c:flash_init() and many
board BSPs as well.

Signed-off-by: Wolfgang Grandegger <wg@grandegger.com>