riscv: configs: Rename ax25-ae350 defconfig

riscv: configs: Rename ax25-ae350 defconfig

Remove cpu name from the defconfig naming.
Because other cpus maybe run on AE350 platform.
So only use platfrom name in defconfig naming
will be better.

Also sync MAINTAINERS:
Rename
a25-ae350_32_defconfig as ae350_rv32_defconfig
ax25-ae350_64_defconfig as ae350_rv64_defconfig

Signed-off-by: Rick Chen <rick@andestech.com>
Cc: Greentime Hu <greentime@andestech.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>