drivers: Add SPMI bus uclass

Qualcom processors use proprietary bus to talk with PMIC devices -
SPMI (System Power Management Interface).
On wiring level it is similar to I2C, but on protocol level, it's
multi-master and has simple autodetection capabilities.
This commit adds simple uclass that provides bus read/write interface.

Signed-off-by: Mateusz Kulikowski <mateusz.kulikowski@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/Kconfig b/drivers/Kconfig
index 70993fd..c82a94b 100644
--- a/drivers/Kconfig
+++ b/drivers/Kconfig
@@ -60,6 +60,8 @@
 
 source "drivers/spi/Kconfig"
 
+source "drivers/spmi/Kconfig"
+
 source "drivers/thermal/Kconfig"
 
 source "drivers/timer/Kconfig"