mb/up/squared: Increase MAX_CPUS from 2 to 4

The board also supports Atom processors, which have four physical cores.

Signed-off-by: Reto Buerki <reet@codelabs.ch>
Change-Id: I98a3da660052eb7ad2f18b0c7fc0e67a609eac54
Reviewed-on: https://review.coreboot.org/c/coreboot/+/44439
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Christian Walter <christian.walter@9elements.com>
Reviewed-by:  Felix Singer <felixsinger@posteo.net>

src/mainboard/up/squared/Kconfig

@@ -62,7 +62,7 @@ config PXE_ROM_ID
 
 config MAX_CPUS
 	int
-	default 2
+	default 4
 
 config LINUX_COMMAND_LINE
 	string