mb/google/fizz: enable eist (enhanced speedstep)

Without eist enabled, fizz's CPU clocks are locked at the
base frequency, and don't scale up or down.  This prevents
fizz from idling properly and turbo boost from functioning,
so enable it (as is done for all other KBL boards)

Test: build/boot google/fizz, ensure CPU clocks scale as expected

Change-Id: I77dd0e1df1bf88f5bae18e9f832ca8d60fb777b4
Signed-off-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-on: https://review.coreboot.org/c/30674
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

src/mainboard/google/fizz/variants/baseboard/devicetree.cb

@@ -7,6 +7,8 @@ chip soc/intel/skylake
 	register "deep_s5_enable_dc" = "1"
 	register "deep_sx_config" = "DSX_EN_LAN_WAKE_PIN | DSX_EN_WAKE_PIN"
 
+	register "eist_enable" = "1"
+
 	# Mapping of USB port # to device
 	#+----------------+-------+-----------------------------------+
 	#| Device         | Port# | Rev                               |