targets/arty_base: use fixed value for clock (100MHz) since generated from PLL and use MiniSoC as default target
targets/arty_base: use fixed value for clock (100MHz) since generated from PLL and use MiniSoC as default target

cr1901/HDMI2USB-litex-firmware,mithro/HDMI2USB-litex-firmware,mithro/HDMI2USB-litex-firmware,mithro/HDMI2USB-litex-firmware,cr1901/HDMI2USB-litex-firmware,mithro/HDMI2USB-litex-firmware,cr1901/HDMI2USB-litex-firmware,cr1901/HDMI2USB-litex-firmware