log: clean up Kconfig

LOG_DEFAULT_LEVEL has been chosen as 6. Adjust the default of LOG_MAX_LEVEL
to this value.

Use ranges to clamp log levels to reasonable values.

Group output options by main U-Boot, SPL, TPL, followed by other logging
options.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>