npcx: i2c: change i2c settings for better timing.
npcx: i2c: change i2c settings for better timing.

The setup time of original i2c setting which i2c freq is 1MHz is closed
to the margin of the spec. This CL improves the setup time with a new
setting. (For example, in npcx evb, measured setup time now is about
480 ns.)
This CL also removes the timing settings of higher i2c source clock
frequencies which are not used so far to save the code size.

BRANCH=none
BUG=b:38217035
TEST=No build error for make buildall; run stress test with i2cxfer and
i2cscan on gru with 400K and 1MHz i2c freq.

Change-Id: I5428a7dab1d935fd428ee9012604813e752cead8
Signed-off-by: CHLin <84d2fca36f9d804bb7b1903655f14e4c722a99c4@nuvoton.com>
Reviewed-on: https://chromium-review.googlesource.com/527739
Commit-Ready: CH Lin <49997681dc186e71092740314bf6c6feeca6e227@nuvoton.com>
Tested-by: CH Lin <49997681dc186e71092740314bf6c6feeca6e227@nuvoton.com>
Reviewed-by: Aaron Durbin <af0622ab041e8b0f8e5819838dcc93898196e71f@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec