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

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)