U-Boot contains a lot of duplicit implementations of serial_puts()
call which just pipes single characters into the port in loop. Implement
function that does this behavior into common code, so others can make
easy use of it.

This function is called default_serial_puts() and it's sole purpose
is to call putc() in loop on the whole string passed to it.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Tom Rini <trini@ti.com>