hw/onenand: program actions can only clear bits

The program actions onenand_prog_main() and onenand_prog_spare()
can only set bits.

This implies a rewrite of onenand_erase() to not use the program
functions, since erase does need to set bits.

Signed-off-by: Juha Riihimäki <juha.riihimaki@nokia.com>
[Riku Voipio: Fixes and restructuring patchset]
Signed-off-by: Riku Voipio <riku.voipio@iki.fi>
[Peter Maydell: More fixes and cleanups for upstream submission]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Andrzej Zaborowski <andrew.zaborowski@intel.com>