include: misc: Add a utility macro to generate a bit mask

Such helper is useful to generate a complete bit mask from a given
number. For instance BIT_MASK(2) will output 0x03, or BIT_MASK(8) will
output 0xFF

Change-Id: I406de767d839b7b2d37024b7b41679edddabe551
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>