On i.MX51/i.MX53 it is necessary to set the DBGEN bit in
ARM_GPC register in order to turn on the debug clocks.
The DBGEN bit of ARM_GPC register has the following description
in the i.MX53 Reference Manual:
"This allows the user to manually activate clocks within the debug
system. This register bit directly controls the platform's dbgen_out
output signal which connects to the DAP_SYS to enable all debug clocks.
Once enabled, the clocks cannot be disabled except by asserting the
disable_trace input of the DAP_SYS."