include: asm: dma-mapping: get rid of the compilation warning in udc-core

Fixed the following warning here.
"warning: ‘dma_alloc_coherent’ defined but not used" while compiling
udc-core

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Reviewed-by: Lukasz Majewski <l.majewski@samsung.com>

arch/arm/include/asm/dma-mapping.h

@@ -14,7 +14,7 @@ enum dma_data_direction {
 	DMA_FROM_DEVICE		= 2,
 };
 
-static void *dma_alloc_coherent(size_t len, unsigned long *handle)
+static inline void *dma_alloc_coherent(size_t len, unsigned long *handle)
 {
 	*handle = (unsigned long)memalign(ARCH_DMA_MINALIGN, len);
 	return (void *)*handle;