mbedtls: add Kconfig symbol CONFIG_MBEDTLS_PSA_STATIC_KEY_SLOTS

Adding the Kconfig symbol CONFIG_MBEDTLS_PSA_STATIC_KEY_SLOTS
to allow Mbed TLS's PSA Crypto core to use static key buffers
to store key's material. This helps reducing heap memory
usage and, potentially, it also discard code implementing
heap memory management if there's no other module in the build
that makes use of it.

Signed-off-by: Valerio Setti <vsetti@baylibre.com>