Reduce default MBEDTLS_MPI_MAX_SIZE
Reduce default MBEDTLS_MPI_MAX_SIZE

Reduce the default size of `MBEDTLS_MPI_MAX_SIZE` to 512 bytes,
as the default 1024 consumes much stack, and supporting RSA 4096 bit
may suffice at the moment.

mbedmicro/mbed,andcor02/mbed-os,andcor02/mbed-os,mbedmicro/mbed,andcor02/mbed-os,kjbracey-arm/mbed,kjbracey-arm/mbed,mbedmicro/mbed,kjbracey-arm/mbed,andcor02/mbed-os,kjbracey-arm/mbed,andcor02/mbed-os,andcor02/mbed-os,mbedmicro/mbed,mbedmicro/mbed