bootcount: add a DM SPI flash backing store for bootcount

This driver allows to use SPI flash as backing store for
boot counter values with DM enabled.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
diff --git a/drivers/bootcount/Makefile b/drivers/bootcount/Makefile
index 059d40d..51d860b 100644
--- a/drivers/bootcount/Makefile
+++ b/drivers/bootcount/Makefile
@@ -12,3 +12,4 @@
 obj-$(CONFIG_DM_BOOTCOUNT)      += bootcount-uclass.o
 obj-$(CONFIG_DM_BOOTCOUNT_RTC)  += rtc.o
 obj-$(CONFIG_DM_BOOTCOUNT_I2C_EEPROM)	+= i2c-eeprom.o
+obj-$(CONFIG_DM_BOOTCOUNT_SPI_FLASH)	+= spi-flash.o