riscv: openisa_rv32m1: Fix booting of rv32m1_vega

rv32m1_vega don't boot due to device init ordering and changes with the
device model.  The soc code is looking for a device pointer for the
intmux.  Change to using DEVICE_DT_GET here as that will ensure we get
a valid pointer and by the time we need to utilize the pointer the
intmux driver will have been initialized and thus the device pointer
will be ready.

Also set BUILD_OUTPUT_HEX since we utilize openocd to flash and west
flash is looking for a hex file for openocd targets.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>