aarch64: smccc: Retrieve up to 8 64-bit values

The most common secure monitor firmware in the ARM world is TF-A. The
current release allows up to 8 64-bit values to be returned from a
SMC64 call from AArch64 state.

Extend the number of possible return values from 4 to 8.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>