kernel.h: split the hexadecimal related helpers to hex.h

For the sake of cleaning up the kernel.h split the hexadecimal related
helpers to own header called 'hex.h'.

Link: https://lkml.kernel.org/r/20230323155029.40000-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>