iPod Classic: YUV to RGB optimizations for ARM v5+

Optimizes YUV to RGB conversion using ARMv5 multiply-accumulate
intructions for operations and data tables for saturation.

This first patch set includes the three versions i have developed.
Although iPod Classic need to use the latest version to reach 30fps,
old versions may serve other targets.

All versions are based on current SVN algorithm (round->scale->add)
using the same coefficients, so output results are identical.

Version history:

  ARMv4:
   - use all available registers to calculate four pixels within each
     loop iteration.
   - avoid LDR interlocks.

  ARMv5TE:
   - use ARMv5TE+ 1-cycle multiply-accumulate instructions.

  ARMv5TE_WST:
   - use data tables (256 bytes) for RBG565 saturation.

Benchmarks results using iPod Classic (ARM926EJ 216Mhz):

                 size    test_fps (1)    mpegplayer (2)
                 bytes   YUV  YUV1/4     average  min/max
                 -----   -----------     ------------------
  SVN-20141107   528     27.8  110.0     11035  10864/13397
  ARMv4          480     28.8  114.0      9767   9586/12126
  ARMv5TE        468     29.7  117.5      8751   8584/11118
  ARMv5TE_WST    544     33.6  133.0      6355   6316/6403

  (1) boosted
  (2) play full elephants_dream_320x240.mpg file (15693 frames) using
      mpegplayer, patched RB measures YUV to RGB565 frame conversion
      time (microseconds)

  Compared against the WST version, the ARMV5TE version w/o cached
  saturation tables is slower, but it is smaller and i have doubts
  about the power consumption.

Change-Id: I2b6a81804636658d85a1bb104ccb2055e77ac120
Reviewed-on: http://gerrit.rockbox.org/1034
Reviewed-by: Cástor Muñoz <cmvidal@gmail.com>
Tested: Cástor Muñoz <cmvidal@gmail.com>