DA830: fix USB 2.0 clock entry

DA8xx OHCI driver fails to load due to failing clk_get() call for the USB 2.0
clock. Arrange matching USB 2.0 clock by the clock name instead of the device.
(Adding another CLK() entry for "ohci.0" device won't do -- in the future I'll
also have to enable USB 2.0 clock to configure CPPI 4.1 module, in which case
I won't have any device at all.)

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>