arch/mips: add support for MIPS NaN

arch/mips: add support for MIPS NaN

MIPS supports two different NaN encodings, legacy and 2008. Information
about MIPS NaN encodings can be found here:

  https://sourceware.org/binutils/docs/as/MIPS-NaN-Encodings.html

NaN legacy is the only option available for R2 cores and older.
NaN 2008 is the only option available for R6 cores.
R5 cores can have either NaN legacy or NaN 2008, depending on the
implementation. So, if the user selects a generic R5 target architecture
variant, we show a choice menu with both options available. For well
known R5 cores we directly select the NaN enconding they use.

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>