ARM: dts: stm32: update uart4 pin configuration for low power on stm32mp157

Sleep pin configuration is refined for low power modes:
- "sleep" (no wakeup & console suspend enabled): put pins in analog state
  to optimize power
- "idle" (wakeup capability): keep Rx pin in alternate function

Signed-off-by: Bich Hemon <bich.hemon@st.com>
Signed-off-by: Erwan Le Ray <erwan.leray@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>