libpayload: Add const for dma_coherent argument

Add the const modifier for the ptr argument of dma_coherent to avoid
unnecessary type casting in payloads.

BUG=none
TEST=emerge-corsola libpayload
BRANCH=none

Change-Id: Ic4bb1d8318c7e83fd3ab3054aa0333cb27afe588
Signed-off-by: Yu-Ping Wu <yupingso@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/66961
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-by: Hsin-Te Yuan <yuanhsinte@google.com>

payloads/libpayload/libc/malloc.c

@@ -118,7 +118,7 @@ int dma_initialized()
 }
 
 /* For boards that don't initialize DMA we assume all locations are coherent */
-int dma_coherent(void *ptr)
+int dma_coherent(const void *ptr)
 {
 	return !dma_initialized() || (dma->start <= ptr && dma->end > ptr);
 }