omap5: Migrate CONFIG_OMAP_PLATFORM_RESET_TIME_MAX_USEC to Kconfig

While in theory this value could be used in places outside of "omap5"
(such as OMAP4), we only make use of it today in OMAP5, so place the
Kconfig entry there.  Given that Kconfig lets us provide a default, we
drop CONFIG_DEFAULT_OMAP_RESET_TIME_MAX_USEC entirely.  The contents of
doc/README.omap-reset-time make a good help entry, so adjust them
slightly and delete the file.  Move the comment about range to where we
use the value now, and have Kconfig enforce the upper bound.

Signed-off-by: Tom Rini <trini@konsulko.com>