[ncp] Add new spinel property MAC_DATA_POLL_PERIOD and its get/set handlers (#1952)

The spinel-protocol draft is also updated.

doc/spinel-protocol-src/spinel-prop-mac.md

@@ -122,6 +122,22 @@ per scanned channel with following format:
 * `C`: Channel
 * `c`: RSSI (in dBm)
 
+### PROP 58: PROP_MAC_DATA_POLL_PERIOD {#prop-mac-data-poll-period
+* Type: Read-Write
+* Packed-Encoding: `L`
+
+The (user-specified) data poll (802.15.4 MAC Data Request) period
+in milliseconds. Value zero means there is no user-specified
+poll period, and the network stack determines the maximum period
+based on the MLE Child Timeout.
+
+If the value is non-zero, it specifies the maximum period between
+data poll transmissions. Note that the network stack may send data
+request transmissions more frequently when expecting a control-message
+(e.g., when waiting for an MLE Child ID Response).
+
+This property is only present on NCPs which implement 802.15.4.
+
 ### PROP 4864: PROP_MAC_WHITELIST  {#prop-mac-whitelist}
 * Type: Read-Write
 * Packed-Encoding: `A(T(Ec))`