x86: dfi-bt700: Adjust CONFIG_TEXT_BASE

At present U-Boot no longer builds as a complete rom for all the
configs of dfi-bt700.

    BINMAN  .binman_stamp
  Wrote map file './rom.map' to show errors
  binman: Section '/binman/rom': contents size 0x80e836 (8448054) exceeds section size 0x800000 (8388608)

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

  <none>     fff00000  000aac90  u-boot-with-ucode-ptr
  <none>     fffa0000  00010000  intel-vga
  <none>     fffaac90  00001df0  u-boot-dtb-with-ucode
  <none>     fffaca80  00019800  u-boot-ucode

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: Stefan Roese <sr@denx.de>