mainboard: add Dell Latitude E7240

Based on autoport output.

It boots to Arch Linux (Linux 6.6.3) from USB and mSATA with SeaBIOS.

Change-Id: I6933bdbcc8d0bbb85d62657624740266284ac71c
Signed-off-by: Iru Cai <mytbk920423@gmail.com>
Signed-off-by: Nicholas Chin <nic.c3.14@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/79746
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>

Documentation/mainboard/index.md

@@ -76,6 +76,7 @@ N130WU / N131WU <clevo/n130wu/index.md>
 ```{toctree}
 :maxdepth: 1
 
+Latitude E7240 <dell/e7240.md>
 OptiPlex 9010 SFF <dell/optiplex_9010.md>
 ```