cpu/efm32: efm32lg: add vendor headers

cpu/efm32/families/efm32lg/include/vendor/efm32lg990f256.h

cpu/efm32/families/efm32lg/include/vendor/efm32lg_acmp.h

cpu/efm32/families/efm32lg/include/vendor/efm32lg_adc.h

cpu/efm32/families/efm32lg/include/vendor/efm32lg_aes.h

cpu/efm32/families/efm32lg/include/vendor/efm32lg_af_pins.h

cpu/efm32/families/efm32lg/include/vendor/efm32lg_af_ports.h

cpu/efm32/families/efm32lg/include/vendor/efm32lg_burtc.h

...efm32/families/efm32lg/include/vendor/efm32lg_burtc_ret.h

...efm32/families/efm32lg/include/vendor/efm32lg_calibrate.h

cpu/efm32/families/efm32lg/include/vendor/efm32lg_cmu.h

cpu/efm32/families/efm32lg/include/vendor/efm32lg_dac.h

cpu/efm32/families/efm32lg/include/vendor/efm32lg_devinfo.h

cpu/efm32/families/efm32lg/include/vendor/efm32lg_dma.h

cpu/efm32/families/efm32lg/include/vendor/efm32lg_dma_ch.h

.../families/efm32lg/include/vendor/efm32lg_dma_descriptor.h

cpu/efm32/families/efm32lg/include/vendor/efm32lg_dmactrl.h

cpu/efm32/families/efm32lg/include/vendor/efm32lg_dmareq.h

cpu/efm32/families/efm32lg/include/vendor/efm32lg_ebi.h

cpu/efm32/families/efm32lg/include/vendor/efm32lg_emu.h

cpu/efm32/families/efm32lg/include/vendor/efm32lg_etm.h