ppc: Add support for 'mffscrn','mffscrni' instructions

ppc: Add support for 'mffscrn','mffscrni' instructions

ISA 3.0B added a set of Floating-Point Status and Control Register (FPSCR)
instructions: mffsce, mffscdrn, mffscdrni, mffscrn, mffscrni, mffsl.
This patch adds support for 'mffscrn' and 'mffscrni' instructions.

'mffscrn' and 'mffscrni' are similar to 'mffsl', except they do not return
the status bits (FI, FR, FPRF) and they also set the rounding mode in the
FPSCR.

On CPUs without support for 'mffscrn'/'mffscrni' (below ISA 3.0), the
instructions will execute identically to 'mffs'.

Signed-off-by: Paul A. Clarke <[email protected]>
Message-Id: <1568817081[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Signed-off-by: David Gibson <[email protected]>