spi: ath79: add mem_ops for fast-read

Reading from memory is 3x faster than bit-bang read operation. Also,
for tl-wr2543nd, the bit-bang read was sporadically returning random
data, possibly a HW defect, while fast-read works as expected.

Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
Link: https://lore.kernel.org/r/20220129040453.8476-1-luizluca@gmail.com


Signed-off-by: Mark Brown <broonie@kernel.org>