Defines MBEDTLS_ECDH_LEGACY_CONTEXT in ecdh.h
Defines MBEDTLS_ECDH_LEGACY_CONTEXT in ecdh.h

Commit adds the conditional definition of
MBEDTLS_ECDH_LEGACY_CONTEXT to ecdh.h.
MBEDTLS_ECDH_LEGACY_CONTEXT is only defined
if MBEDTLS_ECP_RESTARTABLE is definied.

Signed-off-by: Thomas Daubney <5b910c8e7c225fd62319de5b44eecef1f6356fe1@arm.com>

ARMmbed/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls