x86: som-db5800-som-6867: Adjust CONFIG_TEXT_BASE

At present U-Boot no longer builds as a complete rom for som-db5800-som-6867.

    BINMAN  .binman_stamp
  Wrote map file './rom.map' to show errors
  binman: Section '/binman/rom': contents size 0x80302c (8400940) exceeds section size 0x800000 (8388608)

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

  <none>     fff00000  0009f7c8  u-boot-with-ucode-ptr
  <none>     fff90000  00010000  intel-vga
  <none>     fff9f7c8  00001aae  u-boot-dtb-with-ucode

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

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>