drivers: bbram: Introduce STM32 BBRAM driver

STM32 battery-backed RAM is organized in 4 byte registers. Number of
registers can vary between models from 5 to 32 registers.

Usually, the registers are part of RTC. On some variants they are part
of tamper module. On STM32F1 the registers are in separate module. For
now, only backup registers from RTC are supported.

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