arm: debug: add Kconfig entries for lowlevel debug

We have not had a good method to debug the early boot stage such as
lowlevel_init function.  I guess developers generally use dedicated
debuggers for that, but it is difficult in some cases.
(For example, my debugger cannot connect to the ARM processor when
it is in the secure state.  It sometimes happens when I need to
debug the early boot stage on ARM SoCs with secure extension.)

The low level debug feature in Linux would be also helpful for U-boot
when we are stucking in nasty problems where the console is not
available yet.

You have to enable CONFIG_DEBUG_LL to use this feature.
For now, only 8250-compatible UART devices are supported.
You can add a header file under arch/arm/include/debug/ directory
to support your UART device if necessary.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>