device/pciexp: Add support for PCIe CLK power management

Set PCIe "Enable Clock Power Management", if endpoint supports it.

BUG=chrome-os-partner:31424
BRANCH=none
TEST=build and boot on rambi, check Enable Clock Power Management
     in link control register is set properly

Change-Id: Ie54110d1ef42184cfcf47c9fe4d735960aebe47f
Signed-off-by: Kane Chen <kane.chen@intel.com>
Reviewed-on: https://chromium-review.googlesource.com/220742
Reviewed-by: Duncan Laurie <dlaurie@chromium.org>
[Edit commit message.]
Signed-off-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-on: http://review.coreboot.org/8447
Tested-by: build bot (Jenkins)
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/device/Kconfig

@@ -251,6 +251,14 @@ config PCIEXP_ASPM
 	help
 	  Detect and enable ASPM on PCIe links.
 
+config PCIEXP_CLK_PM
+	prompt "Enable PCIe Clock Power Management"
+	bool
+	depends on PCIEXP_PLUGIN_SUPPORT
+	default n
+	help
+	  Detect and enable Clock Power Management on PCIe.
+
 config PCI_BUS_SEGN_BITS
 	int
 	default 0