arm64: dts: mt7986: use size of reserved partition for bl2

arm64: dts: mt7986: use size of reserved partition for bl2

To store uncompressed bl2 more space is required than partition is
actually defined.

There is currently no known usage of this reserved partition.
Openwrt uses same partition layout.

We added same change to u-boot with commit d7bb1099 [1].

[1] https://source.denx.de/u-boot/u-boot/-/commit/d7bb109900c1ca754a0198b9afb50e3161ffc21e

Cc: stable@vger.kernel.org
Fixes: 8e01fb15b815 ("arm64: dts: mt7986: add Bananapi R3")
Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Daniel Golle <daniel@makrotopia.org>
Link: https://lore.kernel.org/r/20230528113343.7649-1-linux@fw-web.de
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>