Add CONFIG_SRAM to allow use of on-chip SRAM on some systems

Add CONFIG_SRAM to allow use of on-chip SRAM on some systems

Some SoCs have a small amount of on-chip SRAM which may be used
for frequently used code or data.  When enabled, this option
will allow FFmpeg to use such facilities where supported.

Originally committed as revision 21564 to svn://svn.ffmpeg.org/ffmpeg/trunk