mbedtls: option for MBEDTLS_PLATFORM_ZEROIZE_ALT

Add a Kconfig option for enabling `MBEDTLS_PLATFORM_ZEROIZE_ALT`, which
is required if the mbed backend already provides an implementation.

For example, the Nordic cryptocell precompiled backends.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>