arm: cortex_a_r: rename z_platform_init to z_arm_platform_init

Platform specific initialization during early boot
has been a feature supported only by Cortex-M; the
Kconfig symbol is define in arch/arm Kconfig space.
We rename the z_platform_init() function to
z_arm_platform_init(), to indicate more clearly that
this is an internal, private ARM-only API.

This commit does not introduce behavioral changes.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>

arch/arm/core/aarch32/cortex_a_r/reset.S

@@ -29,7 +29,7 @@ GDATA(z_arm_fiq_stack)
 GDATA(z_arm_abort_stack)
 GDATA(z_arm_undef_stack)
 #if defined(CONFIG_PLATFORM_SPECIFIC_INIT)
-GTEXT(z_platform_init)
+GTEXT(z_arm_platform_init)
 #endif
 
 /**
@@ -177,7 +177,7 @@ SECTION_SUBSEC_FUNC(TEXT, _reset_section, __start)
 
 #if defined(CONFIG_PLATFORM_SPECIFIC_INIT)
     /* Execute platform-specific initialisation if applicable */
-    bl z_platform_init
+    bl z_arm_platform_init
 #endif
 
 #if defined(CONFIG_WDOG_INIT)