ARM: 7363/1: DEBUG_LL: limit early mapping to the minimum

There is just no point mapping up to 512MB for a serial port.
Using a single 1MB entry is way sufficient for all users.
This will create less interference for the following debugging patch.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>