Crypto: Increase the buffer size for memory allocation

Increase the size of static buffer to 8KB which is used by
Mbed Crypto to dynamically allocate memory. It is required
due to the ECDSA (NIST P-256) signing of initial attestation
token.

Change-Id: I22d916140b0b65fc590de545cf7a086de0acb5cb
Signed-off-by: Tamas Ban <tamas.ban@arm.com>