test: regmap: check the values read from the regmap

The test did reads after writes but didn't check the value.
It probably was because the sandbox didn't implement the writeX/readX
functions.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Updated to use sandbox_set_enable_memio():
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>