MAINTAIERS/printk: Add link to printk git

It might also help to avoid confusion with the historic
pmladek/printk.git that has got obsoleted by printk/linux.git
in February 2020.

Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20220105094157.26216-3-pmladek@suse.com