purism/librem13: Disable L1 sub states support

Some NVMe devices (Intel 600p series for example) seem to lock up
in D3 drive power state (L1.2 PCIe power state).
Disabling L1 substates fixes it.

Change-Id: I00a327dc91d443beb565fe4e72aaf816e40a007c
Signed-off-by: Youness Alaoui <youness.alaoui@puri.sm>
Reviewed-on: https://review.coreboot.org/19900
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

src/mainboard/purism/librem13v1/Kconfig

@@ -21,6 +21,9 @@ config DRIVERS_UART_8250IO
 	  This platform does not have any way to get standard
 	  serial output so disable it by default.
 
+config PCIEXP_L1_SUB_STATE
+	def_bool n
+
 config HAVE_IFD_BIN
 	bool
 	default n