ofwfb: fix incorrect colors on powerpc* and add new tunable parameters

• Implements little-endian support (powerpc64le)
• Adds 'hw.ofwfb.physaddr' kernel parameter so user can manually provide correct address if it's not detected correctly
• Adds 'hw.ofwfb.argb32_pixel' so user can set it manually if colors are inverted due to incorrect pixel format (default = 1)
• Automatically selects RGBA32 pixel format if NVidia graphic adapter is detected (sets hw.ofwfb.argb32_pixel=0)

Machines equipped with NVidia graphic adapters tend to use RGBA32
pixel format. By default ARGB32 pixel format is used, proved to work
on machines equipped with ATI graphic adapter and the onboard adapter
used on Talos II and Blackbird machines from Raptor Computing Systems.

Original patch developed by bdragon

Reviewed by: bdragon, luporl
MFC after: 3 days
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D28604