stm32:l0: Add RCC register definitions

Tested with a miniblink example on the l053 discovery board.

Only register definitions at this stage, no helpers.
Register definitions from RM0367r2, hopefully the biggest
superset of L0 parts.

Signed-off-by: Karl Palsson <karlp@tweak.net.au>
stm32:l0: Add linker script / memory sizes

Easier to pick a corresponding part.