topic: STM32W support added to em357 driver

The em357 driver only supported one page configuration (192k in 96 2048k)
pages. This is fine for em357 chips since that's the size they have, but
ST's STM32W chips (pretty much the same) have different flash
configurations available (64, 128, 192, 256k). I can't find anywhere
that would indicate the size of the chip anywhere in memory so the
selection must be manual, using the 'size' parameter. For backwards
compatibility, any size not known to be in use defaults to the 192k
configuration. I don't have any em357 devices to test, but I also found
that I had to re-assert the FPEC clock enable before performing an
erase. This is a single line and shouldn't break any configurations.

My testing so far has only been with a 64k device with 8k of RAM.

Change-Id: Ic0ac400a9696efaa09d1407dd4a4d456bc2c318b
Signed-off-by: Ben Nahill <bnahill@gmail.com>
Reviewed-on: http://openocd.zylin.com/1336
Tested-by: jenkins
Reviewed-by: Peter Stuge <peter@stuge.se>