lib: add crc7 from Linux

Crc7 is used to compute mmc spi command packet checksum.

Copy from linux-2.6 lib/crc7.c include/linux/crc7.h
commit ad241528c4919505afccb022acbab3eeb0db4d80

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>