gp: update API files to use the imp field in TEEC_Session

The OP-TEE client library has been updated to include an 'imp' field as
required by the GlobalPlatform specification. Therefore, update the
parts of the code that access the TEE context which is now inside this
'imp' struct.
This commit is the result of the following commands:

 sed -i 's/sess->ctx/sess->imp.ctx/' $(git grep -l 'sess->ctx')
 sed -i 's/s->ctx/s->imp.ctx/' $(git grep -l 's->ctx')

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>