armv7m: cache: add mmu_set_region_dcache_behaviour() stub for compatibility

Since some driver requires this function add it as an empty stub
when DCACHE is OFF.

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
diff --git a/arch/arm/cpu/armv7m/cache.c b/arch/arm/cpu/armv7m/cache.c
index f4ba3ad..7353698 100644
--- a/arch/arm/cpu/armv7m/cache.c
+++ b/arch/arm/cpu/armv7m/cache.c
@@ -291,6 +291,12 @@
 void invalidate_dcache_all(void)
 {
 }
+
+void mmu_set_region_dcache_behaviour(phys_addr_t start, size_t size,
+				     enum dcache_option option)
+{
+}
+
 #endif
 
 #if !CONFIG_IS_ENABLED(SYS_ICACHE_OFF)