dts: minor update for nucleo_l476rg

Move SRAM and flash nodes out of stm32l476.dtsi
RAM and Flash sizes depend on last letter ("G" in the case).
Hence it must be defined out of soc description file
Update information for leds

Change-Id: I980129c44c335322ddbe57252a7001296094c7cb
Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>