powerpc: Add DSCR FSCR register bit definition

This sets the DSCR (Data Stream Control Register) in the FSCR (Facility Status
& Control Register).

Also harmonise TAR (Target Address Register) FSCR bit definition too.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>