arch: arm: core: cortex_m: fix cache disabling in init_arch_hw_at_boot

Use the arch-cache functions instead of the sys-cache-functions
in z_arm_init_arch_hw_at_boot to ensure that the caches are
disabled even when CONFIG_CACHE_MANAGEMENT is disabled.

Signed-off-by: Benedikt Schmidt <benedikt.schmidt@embedded-solutions.at>