x86: Divide Intel Atom CPU compatible to x86 and x86_64

atom.dtsi enforces "intel,x86", but it doesn't help us discern if the
platform is 32 or 64-bit. We do that for example in RISC-V and it's
useful from the tooling perspective.

Signed-off-by: Piotr Zierhoffer <pzierhoffer@antmicro.com>
Signed-off-by: Mateusz Hołenko <mholenko@antmicro.com>