sandbox: Use the address in readl/writel() functions

At present these functions do not touch addr, which can raising warnings
about unused variables.

This fixes the following warnings:

sandbox_spl defconfig
drivers/core/regmap.c: In function ‘regmap_read’:
drivers/core/regmap.c:125:12: warning: unused variable ‘ptr’ [-Wunused-variable]
  uint32_t *ptr = map_physmem(map->base + offset, 4, MAP_NOCACHE);
            ^
drivers/core/regmap.c: In function ‘regmap_write’:
drivers/core/regmap.c:134:12: warning: unused variable ‘ptr’ [-Wunused-variable]
  uint32_t *ptr = map_physmem(map->base + offset, 4, MAP_NOCACHE);

Signed-off-by: Simon Glass <sjg@chromium.org>
Fixes: 3bfb8cb4 (dm: regmap: Implement simple regmap_read & regmap_write)

arch/sandbox/include/asm/io.h

@@ -40,12 +40,12 @@ static inline void unmap_sysmem(const void *vaddr)
 phys_addr_t map_to_sysmem(const void *ptr);
 
 /* Define nops for sandbox I/O access */
-#define readb(addr) 0
-#define readw(addr) 0
-#define readl(addr) 0
-#define writeb(v, addr)
-#define writew(v, addr)
-#define writel(v, addr)
+#define readb(addr) ((void)addr, 0)
+#define readw(addr) ((void)addr, 0)
+#define readl(addr) ((void)addr, 0)
+#define writeb(v, addr) ((void)addr)
+#define writew(v, addr) ((void)addr)
+#define writel(v, addr) ((void)addr)
 
 /* I/O access functions */
 int inl(unsigned int addr);