ath79: add read support using spi-mem

ath79: add read support using spi-mem

Reimplements read optimization on top of spi-mem. Similar to
what 461-spi-ath79-add-fast-flash-read.patch used to do with
the dropped flash read interface.

It accelerate only fast-read op reading flash directly from
memory mapped region. 'm25p,fast-read' must be set in order
to use the new spi-mem.

It improved read speed up to 3x on old devices (tplink,tl-wr2543-v1)
while no speed improvement was noticed on newer devices like
(tplink,archer-c7-v2).

Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>