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

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

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>
(cherry picked from commit c7b9adaf85f818d747eeff5145eb4095ccd587fb)

[toddpoynor@google.com: fixup context]
Change-Id: I6c6b5eb93a69f26b97628726eccf91df739d03af
Signed-off-by: Todd Poynor <toddpoynor@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>