arm: cmsis: Convert enable_floating_point to use CMSIS

As a first step towards removing the custom ARM Cortex-M Core code
present in Zephyr in benefit of using CMSIS, this change replaces
the use of the custom core code with CMSIS macros in
enable_floating_point().

Jira: ZEP-1568

Change-id: I544a712bf169358c826a3b2acd032c6b30b2801b
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>