mb/lenovo/sklkbl_thinkpad: Enable ACPI P-state table generation

Set eist_enable to true to call generate_p_state_entries() while writing
ACPI processor nodes to ACPI.

Resolves: https://ticket.coreboot.org/issues/623

Change-Id: Ic8965dddc0f50ac6dbd1b0af81af546aa53fbb58
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/91173
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>

src/mainboard/lenovo/sklkbl_thinkpad/devicetree.cb

@@ -8,6 +8,8 @@ chip soc/intel/skylake
         register "PmConfigSlpSusMinAssert" = "3"        # 500ms
         register "PmConfigSlpAMinAssert" = "3"          # 2s
 
+	# Generate ACPI P-State table
+	register "eist_enable" = "true"
 	device domain 0 on
 		subsystemid 0x17aa 0x225d inherit
 		device ref igpu on