mailbox: mailbox-test: use print_hex_dump_bytes to allow dynamic printk

Reduce the logging from info to debug. Also use print_hex_dump_bytes
instead as it has support for dynamic printk providing options to
conditionally enable/disable these logs.

Cc: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>