powerpc: Fix endianness of flash_block_list in rtas_flash

The function rtas_flash_firmware passes the address of a data structure,
flash_block_list, when making the update-flash-64-and-reboot rtas call.
While the endianness of the address is handled correctly, the endianness
of the data is not.  This patch ensures that the data in flash_block_list
is big endian when passed to rtas on little endian hosts.

Signed-off-by: Thomas Falcon <tlfalcon@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>