util: Add GENMASK helper macro definition

Macro is used to create contiguous bitmask between the
arguments passed to the macro.

BITS_PER_LONG is computed as the multiplication of predefined
macros `__CHAR_BIT__` and `__SIZEOF_LONG__`.

Both gcc and clang support these predefined macros.

With this change, replace the redundant defintions of
GENMASK with the new generic macro available.

Fixes #10843

Suggested-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>
Signed-off-by: Himanshu Jha <himanshujha199640@gmail.com>