ARM: OMAP: fix default sys_ck.rate for boot-time DPLL detection

In the clock init code, the DPLL value set by the bootloader is
queried, but always turns zero due it's parent clock (sys_ck) having
no default rate.  This results in the improper setting of the default
PRCM rate-table entry and any queries of virt_prcm_set rate to return 0.

Following the example of the TI kernel, set the default sys_ck rate to
13MHz.  Could then be overridden in board setup code.

Signed-off-by: Kevin Hilman <khilman@mvista.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>