ARM: 7073/1: debug: augment DEBUG_LL Kconfig help to clarify behaviour

Enabled DEBUG_LL hardcodes the UART address into the kernel and results
in a non-portable kernel image. Since this option is only intended for
use when debugging early boot failures, supporting multiple platforms
in such a configuration is not the intended use-case.

This patch documents this limitation in the DEBUG_LL Kconfig help text,
so that users are aware of the portability restrictions that are associated
with enabling low-level debugging support.

Reported-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>