Move platform_helpers.S to each platform's folder

In practice, all the functions in this file are platform-specific. It is
better to force all platforms to implement than having some sort of weak
function placeholder.

Porting guide updated.

Change-Id: I5beeeb10bec6fe5178b24503d6da8ca66074a8c6
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>
diff --git a/include/plat/common/platform.h b/include/plat/common/platform.h
index d3a486c..3f452ad 100644
--- a/include/plat/common/platform.h
+++ b/include/plat/common/platform.h
@@ -56,6 +56,14 @@
 void tftf_platform_watchdog_set(void);
 void tftf_platform_watchdog_reset(void);
 
+/* Helper that returns a linear core ID from a MPID */
+unsigned int platform_get_core_pos(u_register_t mpid);
+
+/* Crash console functions */
+int plat_crash_console_init(void);
+int plat_crash_console_putc(int c);
+int plat_crash_console_flush(void);
+
 /* Gets a handle for the initialised IO entity */
 void plat_get_nvm_handle(uintptr_t *handle);
 
@@ -153,11 +161,7 @@
  * Optional functions. A default, weak implementation of those functions is
  * provided, it may be overridden by platform code.
  ******************************************************************************/
-unsigned int platform_get_core_pos(unsigned long mpid);
 unsigned long platform_get_stack(unsigned long mpidr);
-int plat_crash_console_init(void);
-int plat_crash_console_putc(int c);
-int plat_crash_console_flush(void);
 /*
  * plat_get_prot_regions: It returns a pointer to a
  * set of regions used to test mem_protect_check.