[POWERPC] PS3: Preallocate bootmem memory for the PS3 FLASH ROM storage driver

Preallocate 256 KiB of bootmem memory for the PS3 FLASH ROM storage driver.
This can be disabled by passing `ps3flash=off' on the kernel command line.

Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>