Blackfin arch: as pointed out by Robert P. J. Day, update the CPU_FREQ name to...

Blackfin arch: as pointed out by Robert P. J. Day, update the CPU_FREQ name to match current Kconfig

Cc: Robert P. J. Day <rpjday@mindspring.com>
Signed-off-by: Mike Frysinger <michael.frysinger@analog.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>