sandbox: enable button

Enable the support of button (driver and command) on sandbox.
Reviewed-by: NSimon Glass <sjg@chromium.org>
Signed-off-by: NPhilippe Reynes <philippe.reynes@softathome.com>

configs/sandbox_defconfig

@@ -108,6 +108,8 @@ CONFIG_AXI_SANDBOX=y
 CONFIG_BOOTCOUNT_LIMIT=y
 CONFIG_DM_BOOTCOUNT=y
 CONFIG_DM_BOOTCOUNT_RTC=y
+CONFIG_BUTTON=y
+CONFIG_BUTTON_GPIO=y
 CONFIG_CLK=y
 CONFIG_CLK_COMPOSITE_CCF=y
 CONFIG_SANDBOX_CLK_CCF=y