drivers/net/r8168: Add customized LED mode

This patch adds a function to program a customized LED setting
for Realtek 81xx family. It reads the settings from devicetree under
target board and programs the setting to offset 0x18 and 0x19.

BUG=b:65437780

TEST=Add customized_leds register in devicetree.cb under target board,
	enable RT8168_SET_LED_MODE flag. Make sure the setting is
	programmed correctly to offset 0x18 and 0x19. Observed the
	LEDs were behaving as expected. Executed suspend/resume and
	the LEDs were still working as expected.

Change-Id: Ib3d4f2cd98ac391e1661a891d604bdd1974d07f6
Signed-off-by: Gaggery Tsai <gaggery.tsai@intel.com>
Reviewed-on: https://review.coreboot.org/21862
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-by: Furquan Shaikh <furquan@google.com>
3 files changed