xen: arm: correct VTCR setting on arm32.

xen: arm: correct VTCR setting on arm32.

1c92a2aaf8c6 "xen: arm: support for up to 48-bit IPA addressing on
arm64" inadvertently changes the VTCR setting for 32-bit from
0x80003558 to 0x80003518, changing the SL0 setting from 0x1 (p2m
starts at L1) to 0x0 (p2m starts at L2).

For some (inexplicable) reason this doesn't cause any issue on
Arndale but it does on the OdroidXU.

Reported-by: Suriyan Ramasami <suriyan.r@gmail.com>
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Julien Grall <julien.grall@linaro.org>
Tested-by: Suriyan Ramasami <suriyan.r@gmail.com>