Initial support for armv5eb on Sheeva.

Disable L2 cache for Sheeva before switch to big-endian mode.

Note that we need to encode this code sequence in little endian,
as u-boot starts kernel in little-endian mode.

XXX
At the moment, this code is unconditionally enabled if CPU_SHEEVA
option is enabled; it does not work for older CPUs.

It would be better to determine CPU type automatically as done for
little-endian variant, but it is too ugly to hand-assemble for
the opposite byte-order.

It may be nice if we have tools something like:

$ nbarm-elf-bswap -s marvell_start -e Lswitch_endian_done netbsd

Tested for OPENBLOCKS_A6 (for CPU_SHEEVA), as well as KUROBOX_PRO
(for !CPU_SHEEVA).

(rin)