hw/mips/boston: don't make flash region 'nomigrate'

Currently we use memory_region_init_rom_nomigrate() to create
the "boston.flash" memory region, and we don't manually register
it with vmstate_register_ram(). This currently means that its
contents are migrated but as a ram block whose name is the empty
string; in future it may mean they are not migrated at all. Use
memory_region_init_ram() instead.

Note that this is a a cross-version migration compatibility break
for the "boston" machine.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Paul Burton <paul.burton@mips.com>
Signed-off-by: Aleksandar Markovic <aleksandar.markovic@mips.com>