sandbox_flattree: enable button support

sandbox_flattree enables QCOM_PMIC_GPIO which now depends on BUTTON. As
a result the button tests now get run, but fail because the ADC and GPIO
button drivers aren't enabled.

Enable them to run the tests for sandbox_flattree.

Reported-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>
diff --git a/configs/sandbox_flattree_defconfig b/configs/sandbox_flattree_defconfig
index 1bd9109..36f384b 100644
--- a/configs/sandbox_flattree_defconfig
+++ b/configs/sandbox_flattree_defconfig
@@ -101,6 +101,9 @@
 CONFIG_SYS_ATA_REG_OFFSET=1
 CONFIG_SYS_ATA_ALT_OFFSET=2
 CONFIG_SYS_ATA_IDE0_OFFSET=0
+CONFIG_BUTTON=y
+CONFIG_BUTTON_ADC=y
+CONFIG_BUTTON_GPIO=y
 CONFIG_CLK=y
 CONFIG_CLK_COMPOSITE_CCF=y
 CONFIG_CLK_K210=y