Adding MPU macros and improving docs

[M-Plichta]

Functional changes:

• There already were macros for MPU_RBAR shareability field (ARM_MPU_SH_NON, ARM_MPU_SH_OUTER and ARM_MPU_SH_INNER), so I added macros for the access permissions and execute-never fields.
• Added some handy functions and macros for better readability, like ARM_MPU_TYPE function and the macro MAIR_ATTR.
• Added input macros for ARM_MPU_ATTR

Docs:

• Changed wording for clarity and consistency with the Architecture Reference Manual.

[M-Plichta]

If the changes are accepted, examples will have to be changes, like the ones in Ref_MPU8.txt, which I'll do in separate pull requests.

[ghost]

Please update version and date in the header section.

[M-Plichta]

Hey @GuentherMartin, thanks for the comment.

What's wrong with the CoreValidation test?
This is my first pull request so I can't quite work out why it's failing.

Thanks

[ghost]

Hello @M-Plichta, there are some problems within the test environment. I will try to run the tests manually.