zedboard: Support full 32MB flash

Update kernel and machine config to use the sholw flash chip now that
the kernel supports "paging" for the flash device.

root@zedboard:~# df -h
Filesystem                Size      Used Available Use% Mounted on
ubi0:qspi-rootfs         23.5M      6.6M     16.9M  28% /
devtmpfs                231.1M         0    231.1M   0% /dev
tmpfs                    64.0K         0     64.0K   0% /media
tmpfs                   251.1M     24.0K    251.1M   0% /run
tmpfs                   251.1M     24.0K    251.1M   0% /var/volatile