Platform: Add SPE platform-specific hw initialization

Adding tfm_spm_hal_post_init() and tfm_spm_hal_post_init_platform()
functions.

Contrarily to SystemInit() intended for a high priority hw
initialization (for example clock and power subsystems), and called
on a very early boot stage from startup code, this function is called
from C code, hence variables and other drivers data are protected from
being cleared up by the C library init.

These functions can be used for initializing platform-specific hw
resources (e.g. IPC, UART) thus freeing up application main() function
from the platform details.

tfm_spm_hal_post_init() function is intended for initialization common
to all platforms, while tfm_spm_hal_post_init_platform() implements
platform-specific initialization.

Signed-off-by: Andrei Narkevitch <ainh@cypress.com>
Change-Id: I7e5c9941c86a1be7fe718d1ebee7427526e8dd83