npcx/i2c: Track configured I2C port speed
npcx/i2c: Track configured I2C port speed

This adds code to the npcx I2C driver to keep track of the speed setting
used to configure the port. This can be used to determine the current
speed setting without decoding chip registers.

BRANCH=none
BUG=b:201039003
TEST=with follow-on patches, switched I2C bus speed between 400 kHz
	and 1 MHz.

Change-Id: Ib7a61e23ec406deff80afead2e2aa6b64578ea1d
Signed-off-by: Caveh Jalali <08e24a66aefda7a55ca7d73f8903e0fb16eec763@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3181502
Reviewed-by: Daisuke Nojiri <fd5f93af191bf7e8f73ea71cc3c0f66b41b1dd49@chromium.org>

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