spi: mpc8xx: Use 16 bit mode for large transfers with even size

On CPM, the RISC core is a lot more efficiant when doing transfers
in 16-bits chunks than in 8-bits chunks, but unfortunately the
words need to be byte swapped.

So, for large tranfers with an even size, allocate a temporary
buffer and byte-swap data before and after transfer.

This change allows setting higher speed for transfer. For instance
on an MPC 8xx (CPM1 comms RISC processor), the documentation tells
that transfer in byte mode at 1 kbit/s uses 0.200% of CPM load
at 25 MHz while a word transfer at the same speed uses 0.032%
of CPM load. This means the speed can be 6 times higher in
word mode for the same CPM load.

For small transfers, the load reduction is not worth the CPU load
required to allocate the temporary buffer, so do it only when data
size is over 64 bytes.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>

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