xilinx_spips: Use memset instead of a for loop to zero registers

Use memset() instead of a for loop to zero all of the registers.

Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: KONRAD Frederic <frederic.konrad@adacore.com>
Reviewed-by: Francisco Iglesias <frasse.iglesias@gmail.com>
Message-id: c076e907f355923864cb1afde31b938ffb677778.1513104804.git.alistair.francis@xilinx.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
