vt_simplefb: support armv7 and U-Boot-placed framebuffer nodes

[seuros]

Summary

Search for simple-framebuffer nodes under both /chosen and the root,
as U-Boot may place them there, and skip disabled nodes.

Honor memory-region, fix address handling on 32-bit arm, and use the
pmap_preboot API for framebuffer mappings before pmap_bootstrap().

[seuros]

sys/arm/arm/machdep.c

118

@imp , i saw that in some place we started to use booleans.