arm: vp3: remove incorrect const in ff_vp3_idct_dc_add_neon declaration

Was missed in aeaf268e52fc11c1f64914a319e0edddf1346d6a when integrating
clear_blocks into the idct.
aarch64: get_cabac inline asm

Based on the x86 branchless get_cabac asm. get_cabac_noinline() gets
approximately 20% faster (no cycle counts available) compared to clang
from Xcode 5.1 beta5. More than 6% faster overall. A part of the overall
speedup might be explained by additional inlining of get_cabac().
arm: get_cabac inline asm

Based on the aarch64 asm. CPU cycle counts on cortex-a9 compared to
gcc 4.8.2:
before: 475 decicycles in get_cabac_noinline, 67106035 runs, 2829 skips
after:  393 decicycles in get_cabac_noinline, 67106474 runs, 2390 skips

Overall speedup is above 2%. Code generated by clang 3.4 is slower on
the same hardware and the relative change is a little larger.
timer: use mach_absolute_time as high resolution clock on darwin

Not guaranteed to be in nanosecond resolution. On iOS 7 the duration
of one tick is 125/3 ns which is still more than an order of magnitude
better then microseconds.

Replace decicycles with the neutral UNITS. Decicycles is strange but
tenths of a nanosecond and unspecific "deci"-ticks for mach_absolute_time
is just silly.
float_dsp: fix errors in documentation