FDTD Solver: `AMREX_RESTRICT` Pointers & References

[ax3l]

Try if adding AMREX_RESTRICT to pointers and references of the finite difference functions that we inline.
Luca can test whether adding AMREX_RESTRICT helps on Fugaku hardware.

We found this in Feb 2022:

• Elements: Restrict Pointers & References BLAST-ImpactX/impactx#59
• https://godbolt.org/z/vofejGsK8

cc @WeiqunZhang