This change brings support for file-related file system RPC requests
from the trusted world into the OP-TEE driver. In a later change,
directory-related file system RPC requests will be supported.

Issue: MTWN-169

Test: Integration testing on astro with optee_test. Be sure to have
the latest patch from upstream that fixes a storage bug in BL32 and the
test storage TA signed/encrypted with the same keys as those in BL32.

Change-Id: I8073c2ea83702f641ce6c3fd8647f3607c52a52a