target-xtensa: xtfpga: attach FLASH to system IO

XTFPGA FLASH is tied to XTFPGA system IO block. It's not very important
for systems with MMU where system IO block is visible at single
location, but it's important for noMMU systems, where system IO block is
accessible through two separate physical address ranges.

Map XTFPGA FLASH to system IO block and fix offsets used for mapping.
Create and initialize FLASH device with series of qdev_prop_set_* as
that's the preferred interface now. Keep initialization in a separate
function.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Reviewed-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
