zinger: (re-)enable the flash prefetch buffer
zinger: (re-)enable the flash prefetch buffer

At 48Mhz, due to the flash accessing requiring 1 Wait-State, we need to
enable the flash prefetch buffer to get the full CPU execution speed.

The bit used to enable flash prefetching was fixed :
crosreview.com/219921 , but the change was overriden by the CL enabling
the STOP mode : crosreview.com/219933

Signed-off-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>

BRANCH=samus
BUG=chrome-os-partner:28335
TEST=plug Zinger into a PD power sink, with the Twinkie protocol
analyzer observe how long Zinger takes to send back GoodCRC after the
Request and SinkCap messages. With the patch, the average delay is going
from 160us to 50us.

Change-Id: I9de1bc10e14485919046c841eea16de3c3b3c800
Reviewed-on: https://chromium-review.googlesource.com/229671
Tested-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>
Reviewed-by: Alec Berg <1a4429eeda29095a12e19a73405f80f5b7f37a63@chromium.org>
Commit-Queue: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>

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