anx74xx: only enable SBU mux in DP mode
anx74xx: only enable SBU mux in DP mode

TEST=manual on reef
BRANCH=none
BUG=none

Change-Id: I714e40bab8400ffc0b1fef703e7aed1ce73739a4
Signed-off-by: Rachel Nancollas <375089e973a88bcaf6019cd9e416663e9022721a@google.com>
Reviewed-on: https://chromium-review.googlesource.com/360337
Commit-Ready: David Hendricks <3b0deacc4b5357f519084f1ea34bd42bf5d34545@chromium.org>
Tested-by: David Hendricks <3b0deacc4b5357f519084f1ea34bd42bf5d34545@chromium.org>
Reviewed-by: David Hendricks <3b0deacc4b5357f519084f1ea34bd42bf5d34545@chromium.org>

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