src/mainboard: Remove unused 'include <stdlib.h>'

Found using following commande:
diff <(git grep -l '#include <stdlib.h>' -- src/) <(git grep -l '
memalign\|malloc\|free' -- src/) |grep -v vendorcode |grep '<'

Change-Id: Ib2ee840a10de5c10d57aa7a75b805ef69dc8da84
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/41241
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/mediatek/mt8173/dramc_pi_calibration_api.c

@@ -2,7 +2,6 @@
 
 #include <device/mmio.h>
 #include <console/console.h>
-#include <stdlib.h>
 #include <soc/addressmap.h>
 #include <soc/dramc_common.h>
 #include <soc/dramc_register.h>