drivers/eeprom: stm32: Enables only for L1 series

Setting EEPROM_STM32 with `default y` under `if SOC_FAMILY_STM32`
overrides `depends on SOC_SERIES_STM32L1X` in EEPROM_STM32
definition.
Then, if ever EEPROM is set in any file (as in
tests/drivers/build_all`), EEPROM_STM32 will be indeed set,
with potential issues on series where driver is not yet correctly
handled.

Fix this by removing EEPROM_STM32 definition in STM32 generic
file and set `default y` along with the `depends on` to keep
it effective.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>