usrp-e100: add ability to set/get default master clock rate from EEPROM
Mboard eeprom map class can parse the setting (4 byte float).
The clock control will try to set the eeprom rate if present,
otherwise or under failure condition, it sets the default.
Updated docs, example, and provided helpful verbose.
I would prefer that users burn the desired rate to the eeprom (and they may too).