logging: fix for 64 bit timestamp + new timestamp format option

Timestamps can be 32 or 64 bit long based on the platform and
build configurations. The proper way to handle these timestamps
is to use the ad-hoc log_timestamp_t variable.
This patch fixes some timestamp's reference which were still
using uint32_t changing them to log_timestamp_t.

Moreover also a new config is added in order to print the
timestamp as the Linux's kernel format. This might be useful
in AMP platforms in which Linux's and Zephyr's logs must be
interleaved in order to get a more comprehensive log solution.

Signed-off-by: Valerio Setti <vsetti@baylibre.com>