intel: mailbox: Mailbox error recovery handling

Attempt to restart the mailbox if the mailbox driver not able
to write any data into the mailbox command buffer.

Signed-off-by: Chee Hong Ang <chee.hong.ang@intel.com>
Change-Id: Ia45291c985844dec9da82839cac701347534d32b
diff --git a/plat/intel/soc/common/include/socfpga_mailbox.h b/plat/intel/soc/common/include/socfpga_mailbox.h
index 59714fd..3a2bf30 100644
--- a/plat/intel/soc/common/include/socfpga_mailbox.h
+++ b/plat/intel/soc/common/include/socfpga_mailbox.h
@@ -127,6 +127,7 @@
 #define MBOX_JOB_ID_CMD(JOB_ID)		(JOB_ID<<24)
 #define MBOX_CMD_LEN_CMD(CMD_LEN)	((CMD_LEN) << 12)
 #define MBOX_INDIRECT(val)		((val) << 11)
+#define MBOX_CMD_MASK(header)		((header) & 0x7ff)
 
 /* RSU Macros */
 #define RSU_VERSION_ACMF		BIT(8)