[TF-A] Support for passing 18 parameters to/from an SMC call in AArch64

I noticed TF-A currently supports passing in 4 parameters and returning up to 8. But SMCCC 1.1+ supports passing up to 18 and returning 18 in AArch64 mode, and passing in/out 8 in AArch32.

I was wondering if there are any plans to add support for handling the full set of parameters?