serial: stm32: Increase maximum number of ports

Increase max number of ports for stm32h7
which supports up to 8 uart and usart instances.

Signed-off-by: Gerald Baeza <gerald.baeza@st.com>
Signed-off-by: Bich Hemon <bich.hemon@st.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>