Add serial port bitbanging code

This adds the pony_spi driver which supports the SI_Prog adapter, which
is commonly used for SPI chips with PonyProg 2000, and a custom adapter
called "SERBANG" which differs in the logic of two pins.

Corresponding to flashrom svn r1525.

Signed-off-by: Virgil-Adrian Teaca <darkstarlinux@gmail.com>
Acked-by: Michael Karcher <flashrom@mkarcher.dialup.fu-berlin.de>