soc: renesas: rcar-sysc: Improve SYSC interrupt config in legacy wrapper

Align SYSC interrupt configuration in the legacy wrapper with the DT
version:
  - Mask SYSC interrupt sources before enabling them (doesn't matter
    much as they're disabled at the GIC level anyway),
  - Make sure not to clear reserved SYSCIMR bits that were set before.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>