Support contiguous mapping of non-contiguous physical pages

[sangho2]

Although LiteBox kernel platforms (litebox_platform_linux_kernel and litebox_platform_lvbs) use MappedPageTable, this is effectively OffsetPageTable since they use a fixed GVA_OFFSET to implement pa_to_va() and va_to_pa(). These functions are simple and efficient but prevent us from implementing some kernel functions like vmap() and vunmap() #555. Instead of relying on GVA_OFFSET, we need a programmatic way to provide hints for pa to va mappings and should globally maintain their mappings to implement pa_to_va() and va_to_pa() in a dynamic manner.