Add a new error code for lock status

As per the ARC review feedback, it was suggested that the supervisor
software should be able to distinguish between denial reasons if it
was due to locked.

While at it, add some clarification about set operation when the
requested and existing values are same.

Signed-off-by: Atish Patra <[email protected]>

src/binary-encoding.adoc

@@ -59,6 +59,7 @@ error codes.
 |  SBI_ERR_BAD_RANGE         | -11   | Bad (or invalid) range
 |  SBI_ERR_TIMEOUT           | -12   | Failed due to timeout
 |  SBI_ERR_IO                | -13   | Input/Output error
+|  SBI_ERR_DENIED_LOCKED     | -14   | Denied or not allowed due to lock status
 |===
 
 An `ECALL` with an unsupported SBI extension ID (*EID*) or an unsupported SBI

src/ext-firmware-features.adoc

@@ -116,6 +116,9 @@ firmware feature to be set according to the `value` and `flags` parameters for
 which per feature supported values are described in
 <<table_fw_features_attribute_values>> and flags in <<table_fw_features_flags>>.
 
+NOTE: The set operation will succeed if requested `value` matches the existing
+value.
+
 [#table_fw_features_flags]
 .FWFT Firmware Features Set Flags
 [cols="1,1,3", width=90%, align="center", options="header"]
@@ -143,9 +146,10 @@ returned in `sbiret.error` are shown in <<table_fw_features_set_errors>> below.
 | SBI_ERR_INVALID_PARAM | Provided `value` or `flags` parameter is invalid.
 | SBI_ERR_DENIED        | `feature` set operation failed because either: +
                           - it was denied by the SBI implementation +
-                          - `feature` is locked +
                           - `feature` is reserved or is platform-specific and
                           unimplemented
+| SBI_ERR_DENIED_LOCKED | `feature` set operation failed because the `feature`
+			  is locked
 | SBI_ERR_FAILED        | The set operation failed for unspecified or unknown
                           other reasons.
 |===