lib: import bitrev library from the linux kernel

This patch adds the bitrev library from the linux kernel.  This is a simple
algorithm that uses an 8 bit look-up table to reverse the bits in data types of
8, 16, or 32 bit widths.  The docg4 nand flash driver uses it.

[port from linux kernel v3.9 commit 7ee32a6d30d1c8a3b7a07a6269da8f0a08662927]
[originally added: v2.6.20 by commit a5cfc1ec58a07074dacb6aa8c79eff864c966d12]

Signed-off-by: Mike Dunn <mikedunn@newsguy.com>