arm64: entry: Add exception trampoline page for exceptions from EL0

arm64: entry: Add exception trampoline page for exceptions from EL0

commit c7b9adaf85f8 upstream.

To allow unmapping of the kernel whilst running at EL0, we need to
point the exception vectors at an entry trampoline that can map/unmap
the kernel on entry/exit respectively.

This patch adds the trampoline page, although it is not yet plugged
into the vector table and is therefore unused.

Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Laura Abbott <labbott@redhat.com>
Tested-by: Shanker Donthineni <shankerd@codeaurora.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Alex Shi <alex.shi@linaro.org>
Conflicts:
	add asm/mmu.h in entry.S for ASID marco
	add kernel-pgtable.h in entry.S for SWAPPER_DIR_SIZE and
	RESERVED_TTBR0_SIZE
	no SW PAN in vmlinux.lds.S