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.
stm32:l0: RCC: add osc_on/osc_off helpers

These are the routines that have custom switch cases, and aren't easy targets
for pulling out.
stm32/f0: rcc.c Use common version instead of duplicate code.

Pulls out duplicate calls from f0/rcc.c and uses the common version which
also means that f0 can use rcc_peripheral_enable() now which is in common
but not the old rcc versions.