lib/vsprintf: Allow to override ISO 8601 date and time separator

ISO 8601 defines 'T' as a separator between date and time. Though,
some ABIs use time and date with ' ' (space) separator instead.

Add a flavour to the %pt specifier to override default separator.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20210511153958.34527-1-andriy.shevchenko@linux.intel.com