mpc85xx: Make AP3825i boot env partition writable

mpc85xx: Make AP3825i boot env partition writable

End-users may need to be able to rewrite u-boot configuration on the
WS-AP3825i, which has had repeated issues with the exact configuration
of u-boot, e.g. commit 1d06277407 ("mpc85xx: Fix output location of
padded dtb") (alongside other failures documented for example in this
post[^1] from the main AP3825i porting thread).

To assist with this, remove the `read-only` property from the u-boot
configuration partitions cfg1 and cfg2.

[^1]: https://forum.openwrt.org/t/adding-openwrt-support-for-ws-ap3825i/101168/107

Signed-off-by: Martin Kennedy <hurricos@gmail.com>