arch: arm: aarch32: clear CPACR for every CPU that has a FPU

Upon reset, the Co-Processor Access Control Register is, normally,
0x00000000. However, it might be left un-cleared by firmware running
before Zephyr boot.
This restores the register back to reset value, even if CONFIG_FLOAT
is not set.
Clearing before setting supports switching between Full access
and Privileged access only.

Refactor enable_floating_point to support initialize
floating point registers for every CPU that has a FPU.

Signed-off-by: Luuk Bosma <l.bosma@interay.com>