src: Remove unused '#include <stddef.h>'

Unused includes found using following commande:
diff <(git grep -l '#include <stddef.h>' -- src/) <(git grep -l
'size_t\|ssize_t\|wchar_t\|wint_t\|NULL\|DEVTREE_EARLY\|DEVTREE_CONST\
|MAYBE_STATIC_NONZERO\|MAYBE_STATIC_BSS\|zeroptr' -- src/)|grep '<'
|grep -v vendor |grep -vF '.h'

Change-Id: Ic54b1db995fe7c61b416fa5e1c4022238e4a6ad5
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/41150
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/mediatek/common/cbmem.c

@@ -2,7 +2,6 @@
 
 #include <cbmem.h>
 #include <commonlib/helpers.h>
-#include <stddef.h>
 #include <symbols.h>
 #include <soc/emi.h>
 

src/soc/mediatek/common/ddp.c

@@ -2,7 +2,6 @@
 
 #include <device/mmio.h>
 #include <edid.h>
-#include <stddef.h>
 #include <soc/addressmap.h>
 #include <soc/ddp.h>
 

src/soc/mediatek/common/mtcmos.c

@@ -1,6 +1,5 @@
 /* SPDX-License-Identifier: GPL-2.0-only */
 
-#include <stddef.h>
 
 #include <device/mmio.h>
 #include <soc/mtcmos.h>