arm: meson64: enable console mux and console env by default

With the recent addition of the meson VPU driver, enable the following
config entries by default for meson-64 targets: CONFIG_CONSOLE_MUX,
CONFIG_SYS_CONSOLE_IS_IN_ENV.

This allows outputting the console via video if CONFIG_VIDEO_MESON is
selected.

Signed-off-by: Maxime Jourdan <mjourdan@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Anatolij Gustschin <agust@denx.de>