We cannot get a constexpr definition of the register addresses, and any
reinterpret cast is currently illegal for a constexpr in c++17.
Change the SPI0, TIFR and TIMSK to volatile const pointers instead.
SPI0 has volatile members instead of marking the entire struct as
volatile, which is probably not a good idea as it technically drops the
volatile from the original pointer.
Introduces a nasty hack to forcefully write into the constexpr SPI descriptor's registers
(which is the correct way in ASM, but kind of cumbersome in C++ now)
Yuri D'Elia
D.R.racer
Alex Voinea