C6X: fix timer64 initialization

Some SoCs have a timer block enable controlled through the DSCR registers.
There is a problem in the timer64 driver initialization where the code
accesses a timer register to get the divisor used to calculate timer clock
rate. If the timer block has not been enabled when this register read takes
place, an exception is generated. This patch makes sure that the timer block
is enabled before accessing the registers.

Signed-off-by: Mark Salter <msalter@redhat.com>