fsl_imx*: Migrate ROM contents

The fsl-imx* boards accidentally forgot to register the ROM memory
regions for migration.  This used to require a manual step of calling
vmstate_register_ram(), but following commits
1cfe48c1ce21..b08199c6fbea194 we can use memory_region_init_rom() to
have it do the migration for us.

This is a migration break, but the migration code currently does not
handle the case of having two RAM regions which were not registered
for migration, and so prior to this commit a migration load would
always fail with:
  "qemu-system-arm: Length mismatch: 0x4000 in != 0x18000: Invalid argument"

NB: migration appears at this point to be broken for this board
anyway -- it succeeds but the destination hangs; probably some
device in the system does not yet support migration.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1500309775-18361-1-git-send-email-peter.maydell@linaro.org
