bus: ti-sysc: Add initial support for external resets

Some modules need to use external resets in the rstctrl bits. Typically
only one of the rstctrl bits is for the interconnect target module while
the others are for various child devices.

For ti-sysc driver, we just need the module rstctrl bit mapped. The rest
of the rstctrl bits can be directly mapped to the child devices.

Cc: Suman Anna <s-anna@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>