dts/arm/st/f1: Add support for F103xG

* Add ADC2 and ADC3 to the F103xE definition.
* Add a dtsi file for the F103xG based on the F103xE.
The F103xG has 1 MB of flash, 96 KB of RAM, and additional timers
TIM9/10/11/12/13/14.

Signed-off-by: Paul Fagerburg <pfagerburg@google.com>