85xx: Fix the boot window issue

If one custom board is using the 8MB flash, it is set
as FLASH_BASE = 0xef000000, TEXT_BASE = 0xef780000.
The current start.S code will be broken at switch_as.

It is because the TLB1[15] is set as 16MB page size,
EPN = TEXT_BASE & 0xff000000, RPN = 0xff000000.

For the 8MB flash case, the EPN = 0xefxxxxxx,
RPN = 0xffxxxxxx. Assume the virt address of switch_as
is 0xef7ff18c, the real address of the instruction at
switch_as should be 0xff7ff18c. the 0xff7ff18c is out
of the range of the default 8MB boot LAW window
0xff800000 - 0xffffffff.

So when we switch to AS1 address space at switch_as,
the core can't fetch the instruction at switch_as any
more. It will cause broken issue.

Signed-off-by: Dave Liu <daveliu@freescale.com>