spmi: Add sandbox test driver

This patch adds emulated spmi bus controller with part of
pm8916 pmic on it to sandbox and tests validating SPMI uclass.

Signed-off-by: Mateusz Kulikowski <mateusz.kulikowski@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/spmi/Kconfig b/drivers/spmi/Kconfig
index 0b9bd31..c70d675 100644
--- a/drivers/spmi/Kconfig
+++ b/drivers/spmi/Kconfig
@@ -7,4 +7,12 @@
 	  Select this to enable to support SPMI bus.
 	  SPMI (System Power Management Interface) bus is used
 	  to connect PMIC devices on various SoCs.
+
+config SPMI_SANDBOX
+	boolean "Support for Sandbox SPMI bus"
+	depends on SPMI
+	---help---
+	  Demo SPMI bus implementation. Emulates part of PM8916 as single
+          slave (0) on bus. It has 4 GPIO peripherals, pid 0xC0-0xC3.
+
 endmenu