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>