dt-bindings: tegra186-hsp: add type for shared mailboxes

Tegra234 supports sending/receiving 32-bit and 128-bit data over
a shared mailbox. Based on the data size to be used, clients need
to specify the type of shared mailbox in the device tree.

Add a macro for 128-bit shared mailbox. Mailbox clients can use this
macro as a flag in device tree to enable 128-bit data support for a
shared mailbox.

Signed-off-by: Kartik <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Jassi Brar <[e-mail]>

Documentation/devicetree/bindings/mailbox/nvidia,tegra186-hsp.yaml

···
   second cell is used to identify the mailbox that the client is going
   to use.
 
+  For shared mailboxes, the first cell composed of two fields:
+    - bits 15..8:
+        A bit mask of flags that further specifies the type of shared
+        mailbox to be used (based on the data size). If no flag is
+        specified then, 32-bit shared mailbox is used.
+    - bits 7..0:
+        Defines the type of the mailbox to be used. This field should be
+        TEGRA_HSP_MBOX_TYPE_SM for shared mailboxes.
+
   For doorbells, the second cell specifies the index of the doorbell to
   use.
 

include/dt-bindings/mailbox/tegra186-hsp.h

···
 #define TEGRA_HSP_MBOX_TYPE_AS 0x3
 
 /*
+ * These define the types of shared mailbox supported based on data size.
+ */
+#define TEGRA_HSP_MBOX_TYPE_SM_128BIT (1 << 8)
+
+/*
  * These defines represent the bit associated with the given master ID in the
  * doorbell registers.
  */