ast-bmc: Document BMC scratch register

ASPEED BMCs use SIO register 0x29 to configure host firmwrae settings.
This documents those setings as currently used by Hostboot in [1].
Despite the naming, these settings are relevant for ast2500 systems as
well.

[1] src/usr/initservice/bootconfig/bootconfig_ast2400.H

Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Stewart Smith <stewart@linux.ibm.com>

hw/ast-bmc/ast-io.c

@@ -95,6 +95,26 @@
 #define BMC_SIO_SCR28 0x28
 #define BOOT_FLAGS_VERSION 0x42
 
+/*
+ *  SIO Register 0x29: Boot Flags (normal bit ordering)
+ *
+ *       [7:6] Hostboot Boot mode:
+ *              00 : Normal
+ *              01 : Terminate on first error
+ *              10 : istep mode
+ *              11 : reserved
+ *       [5:4] Boot options
+ *              00 : reserved
+ *              01 : Memboot
+ *              10 : Clear gard
+ *              11 : reserved
+ *       [ 3 ] BMC mbox PNOR driver
+ *       [2:0] Hostboot Log level:
+ *                 000 : Normal
+ *                 001 : Enable Scan trace
+ *                 xxx : reserved
+ */
+
 #define BMC_SIO_SCR29 0x29
 #define BMC_SIO_SCR29_MEMBOOT 0x10