dt-bindings: bus: ti-sysc: Add support for PRUSS SYSC type

The PRUSS module has a SYSCFG which is unique. The SYSCFG
has two additional unique fields called STANDBY_INIT and
SUB_MWAIT in addition to regular IDLE_MODE and STANDBY_MODE
fields. Add the bindings for this new sysc type.

Cc: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Suman Anna <s-anna@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>