arm64: entry: Move the trampoline to be before PAN

The trampoline page tables are positioned after the early page tables in
the kernel linker script.

As we are about to change the early page table logic to resolve the
swapper size at link time as opposed to compile time, the
SWAPPER_DIR_SIZE variable (currently used to locate the trampline)
will be rendered unsuitable for low level assembler.

This patch solves this issue by moving the trampoline before the PAN
page tables. The offset to the trampoline from ttbr1 can then be
expressed by: PAGE_SIZE + RESERVED_TTBR0_SIZE, which is available to the
entry assembler.

Tested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Steve Capper <steve.capper@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>