MAINTAINERS: Add maintainers for the cross toolchain

Add the following people as maintainers for the cross toolchain.

  * Martin Roth
  * Felix Singer
  * Elyes Haouas

Change-Id: I3fad10baa0f0177693e009a4bbc218c6064611b9
Signed-off-by: Felix Singer <felixsinger@posteo.net>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/70923
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-by: Martin L Roth <gaumless@gmail.com>

MAINTAINERS

@@ -956,6 +956,10 @@ F:	util/xcompile/
 F:	util/genbuild_h/
 
 TOOLCHAIN
+M:	Martin Roth <gaumless@gmail.com>
+M:	Felix Singer <felixsinger@posteo.net>
+M:	Elyes Haouas <ehaouas@noos.fr>
+S:	Supported
 F:	util/crossgcc/
 
 DOCKER