dts: arm: st: f3: Add RTC BBRAM to DTS

All supported STM32F3 devices have 4-byte battery-backed RTC registers,
but they have different number of registers:

STM32F303x8 and STM32F334 have 5 registers.
STM32F303xc and STM32F303xe have 16 registers.
STM32F302x8 has 20 registers.
STM32F373 has 32 registers.

Signed-off-by: Patryk Duda <pdk@semihalf.com>