dts: silabs: use 'aliases' to remove dts_fixup defines

By adding 'aliases' node in SoC .dtsi file it is possible to generate
DT_ defines which specify a logical name rather than relay on module
location on APB bus. E.g. DT_SILABS_GECKO_USART_40010000_LABEL becomes
DT_SILABS_GECKO_USART_USART_0_LABEL. Thus it is possible to remove
dts_fixup.h defines.

Signed-off-by: Piotr Mienkowski <piotr.mienkowski@gmail.com>