soc: ti: pruss: Refactor the CFG sub-module init

The CFG sub-module is not present on some earlier SoCs like the
DA850/OMAPL-138 in the TI Davinci family. Refactor out the CFG
sub-module parse and initialization logic into a separate function
to make it easier to add logic for the PRUSS IP on the above legacy
SoC families.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>