arm: 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 defined 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>

soc/arm/renesas_rcar/gen3/soc.c

@@ -27,7 +27,7 @@ static int soc_init(const struct device *arg)
 	return 0;
 }
 
-void z_platform_init(void)
+void z_arm_platform_init(void)
 {
 	L1C_DisableCaches();
 	L1C_DisableBTAC();