checkpatch: add support for floating point constants

Even though the kernel doesn't support using floating point constants,
add a regex for them.

Support forms like: 0x123p1, 123e-1, 1.23, 1.5e23f

Signed-off-by: Joe Perches <joe@perches.com>
Cc: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>