spl: Allow mailbox drivers to be used within SPL

Add an option for building mailbox drivers within SPL.

Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
diff --git a/drivers/Makefile b/drivers/Makefile
index d296354..c012476 100644
--- a/drivers/Makefile
+++ b/drivers/Makefile
@@ -14,6 +14,7 @@
 obj-$(CONFIG_$(SPL_TPL_)SPI_FLASH_SUPPORT) += mtd/spi/
 obj-$(CONFIG_$(SPL_TPL_)SPI_SUPPORT) += spi/
 obj-$(CONFIG_$(SPL_TPL_)TIMER) += timer/
+obj-$(CONFIG_$(SPL_)DM_MAILBOX) += mailbox/
 
 ifndef CONFIG_TPL_BUILD
 ifdef CONFIG_SPL_BUILD