soc: st: stm32: stm32g0x: add soc configs for i2c shared irq

check if multiples UART instances with same irq are enabled
at same time then enable shared_interrupt handler.

set the default value of SHARED_IRQ_MAX_NUM_CLIENTS config if
we have more than 2 usarts instances enabled.

Signed-off-by: Fabrice DJIATSA <fabrice.djiatsa-ext@st.com>