mailbox: add Tegra186 HSP driver

Tegra186's HSP module implements doorbells, mailboxes, semaphores, and
shared interrupts. This patch provides a driver for HSP, and hooks it
into the mailbox API. Currently, only doorbells are supported.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/mailbox/Makefile b/drivers/mailbox/Makefile
index bbae4de..155dbeb 100644
--- a/drivers/mailbox/Makefile
+++ b/drivers/mailbox/Makefile
@@ -5,3 +5,4 @@
 obj-$(CONFIG_DM_MAILBOX) += mailbox-uclass.o
 obj-$(CONFIG_SANDBOX_MBOX) += sandbox-mbox.o
 obj-$(CONFIG_SANDBOX_MBOX) += sandbox-mbox-test.o
+obj-$(CONFIG_TEGRA_HSP) += tegra-hsp.o