Core: Refactor tfm_core_ns_ipc_request() function

This patch refactors the tfm_core_ns_ipc_request()
function in order to minimise the inline assembly
required. This improves maintainability and reduces
the potential of incorrect code generation when
aggressive optimisation modes are used during
compilation.

Change-Id: I871f49a4d9c3cf07ebaf29246855c5265b91773c
Signed-off-by: Antonio de Angelis <antonio.deangelis@arm.com>