Mailbox: Use spinlock for non-secure only mailbox data

tfm_ns_mailbox_hal_enter_critical_isr lock access to mailbox data
between cores. This patch use spinlock to lock access to data that are
not shared with secure side to remove unecessary impact on secure core.
Makes code consistent with mailbox_wait_reply_signal.

Signed-off-by: Roman Mazurak <roman.mazurak@infineon.com>
Change-Id: I58b9533f832dab24ffcb3e3b5de5a51a96a0af89