volteer: Remove dependency on npcx specific registers
volteer: Remove dependency on npcx specific registers

This change removes the dependency on the npcx specific headers
which are normally included via registers.h. It instead transitions
to relying on i2c/i2c.h which defines various enums and the
NAMED_I2C macro.

BUG=b:175249000
TEST=zmake testall

Cq-Depend: chromium:2582819
Change-Id: I7d8e98cc4228496b0c7603c0794eb92e0f79c01d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/zephyr-chrome/+/2583272
Tested-by: Jack Rosenthal <d3f605bef1867f59845d4ce6e4f83b8dc9e4e0ae@chromium.org>
Reviewed-by: Jack Rosenthal <d3f605bef1867f59845d4ce6e4f83b8dc9e4e0ae@chromium.org>
Commit-Queue: Jack Rosenthal <d3f605bef1867f59845d4ce6e4f83b8dc9e4e0ae@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2630155
Reviewed-by: Simon Glass <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>

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