drivers/generic/bayhub: Add driver for BayHub BH720

Add a driver which puts the device into power-saving mode.

BUG=b:73726008
BRANCH=none
TEST=boot and see this message:
BayHub BH720: Power-saving enabled 110103
From linux:
$ iotools pci_read32 2 0 0 0x90
0x00110103

Change-Id: Idbfb114f3782c9386ce9b487c3abdb0afbc4a0d9
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-on: https://review.coreboot.org/25966
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
5 files changed