arm64: Extend the idmap to the whole kernel image

This patch changes the idmap page table creation during boot to cover
the whole kernel image, allowing functions like cpu_reset() to be safely
called with the physical address.

This patch also simplifies the create_block_map asm macro to no longer
take an idmap argument and always use the phys/virt/end parameters. For
the idmap case, phys == virt.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>