ffmpeg: disable MIPS DSP options by default

DSP support depends on the core implementation, not on the ISA, so
enabling it for mips64r2 is not correct since you can build a mips64r2
core without DSP support.

Disable mipsdspr1 and mipsdspr2 options by default and let the user
enable them via the BR2_PACKAGE_FFMPEG_EXTRACONF kconfig option, which
will take preference and override the default values.

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>