hw/xtensa/xtfpga: fix FLASH mapping to boot region for KC705

On KC705 bootloader area is located at FLASH offset 0x06000000, not 0 as
on older xtfpga boards.

Cc: qemu-stable@nongnu.org
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
(cherry picked from commit 37ed7c4b)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>