plat: add NSPE platform-specific hw initialization

 Adding tfm_ns_platform_init() function.

 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.

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

 Implemented as a "weak" functions, it can be overwritten by a platform
 specific implementation.

Signed-off-by: Andrei Narkevitch <ainh@cypress.com>
Change-Id: I16091d35a77ba3e246692cff9df7f31ea082ab27