cmsis: Remove cmsis.h

CMSIS-Core is designed to be consumed by including the platform-specific
"Device Header file", named after the concrete platform. The generic
code can depend on a preprocessor macro `CMSIS_device_header` to use the
device selected in a project.

Use CMSIS_device_header instead of cmsis.h to improve code readability.

Signed-off-by: Jaeden Amero <jaeden.amero@arm.com>
Signed-off-by: Filip Jagodzinski <filip.jagodzinski@arm.com>