Add plat_crash_console_flush to platforms without it

Even though at this point plat_crash_console_flush is optional, it will
stop being optional in a following patch.

The console driver of warp7 doesn't support flush, so the implementation
is a placeholder.

TI had ``plat_crash_console_init`` and ``plat_crash_console_putc``, but
they weren't global so they weren't actually used. Also, they were
calling the wrong functions.

imx8_helpers.S only has placeholders for all of the functions.

Change-Id: I8d17bbf37c7dad74e134c61ceb92acb9af497718
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>