x86: minnowmax: Adjust CONFIG_TEXT_BASE

At present U-Boot no longer builds as a complete rom for minnowmax.

    BINMAN  .binman_stamp
  Wrote map file './rom.map' to show errors
  binman: Section '/binman/rom': contents size 0x803146 (8401222) exceeds section size 0x800000 (8388608)

Checking rom.map we see 'fdtmap' section is overlapped with
'intel-vga' section:

  <none>     fffa1390  00019800  u-boot-ucode
  <none>     fffb0000  00010000  intel-vga
  <none>     fffbab90  00000539  fdtmap

Let's adjust CONFIG_TEXT_BASE to allow more space for U-Boot codes.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>