[Nouveau] NV_PMC_BOOT_0 architecture field

[Andy Ritger]

Hi Ben,

At XDC you asked about the architecture field in NV_PMC_BOOT_0, and how
to correctly determine the GPU architecture.

It looks like Nouveau is getting the architecture from bits 20-27 
in NV_PMC_BOOT_0 (though masking off bits 20-23).  For >= NV10, the
architecture field in NV_PMC_BOOT_0 is bits 24-28.  E.g., the GK2xx
architecture has architecture value 0x10, while GK11x has value 0xF and
GK10x has value 0xE.

I hope that helps,
- Andy