README: Clarify difference of CONFIG_WATCHDOG and CONFIG_HW_WATCHDOG

Now that we have the documentation, the code should be changed to reflect
it ;)

Asd far as I can see, these are the places where HW_WATCHDOG is used
instead of WATCHDOG:

arch/blackfin/cpu/blackfin/watchdog.c
arch/m68k/cpu/mcf547x_8x/cpu.c

The relevant maintainers are on CC.

Signed-off-by: Detlev Zundel <dzu@denx.de>
Cc: Mike Frysinger <vapier@gentoo.org>
Cc: TsiChungLiew <Tsi-Chung.Liew@freescale.com>