boards/arduino/portenta_h7: enable USART1 on M4

Right now, USART1 is enabled on the M7 target variant by default, leaving
M4 without a UART to use; this is the way this port was originally
contributed.

Since then, USB was enabled on M7, changing the console backend from USART1
to USB CDC ACM; the M4 target was left unchanged.

This commit enabled USART1 on the M4 variant and disabled it on the M7
variant, so that the M4 variant can use it as its console backend.

Note that, for the M4 variant, USART1 has been assigned to `zephyr,console`
and `zephyr,shell-uart` since this port was contributed, even though USART1
was always disabled on M4.

Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>