arm64: introduce KIMAGE_VADDR as the virtual base of the kernel region

arm64: introduce KIMAGE_VADDR as the virtual base of the kernel region

This introduces the preprocessor symbol KIMAGE_VADDR which will serve as
the symbolic virtual base of the kernel region, i.e., the kernel's virtual
offset will be KIMAGE_VADDR + TEXT_OFFSET. For now, we define it as being
equal to PAGE_OFFSET, but in the future, it will be moved below it once
we move the kernel virtual mapping out of the linear mapping.

Reviewed-by: Mark Rutland <[email protected]>
Signed-off-by: Ard Biesheuvel <[email protected]>
Signed-off-by: Catalin Marinas <[email protected]>