arm64: dts: meson-axg: fix mailbox address

MHU mailbox address is wrong. Fixing it enables the mailboxes on the A113.
These mailboxes are needed for SCPI

Fixes: 9d59b708 ("arm64: dts: meson-axg: add initial A113D SoC DT support")
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>