libflash/file: greatly increase perf of file_erase()
libflash/file: greatly increase perf of file_erase()

Do 4096 byte chunks not 8 byte chunks. A ffspart invocation constructing
a 64MB PNOR goes from a couple of seconds to ~0.1seconds with this
patch.

Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.ibm.com>
Reviewed-by: Samuel Mendoza-Jonas <f16bed56189e249fe4ca8ed10a1ecae60e8ceac0@mendozajonas.com>
Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.ibm.com>

qemu/skiboot,open-power/skiboot,legoater/skiboot,stewart-ibm/skiboot,open-power/skiboot,qemu/skiboot,stewart-ibm/skiboot,shenki/skiboot,legoater/skiboot,shenki/skiboot,shenki/skiboot,stewart-ibm/skiboot,legoater/skiboot,shenki/skiboot,open-power/skiboot,legoater/skiboot,qemu/skiboot,shenki/skiboot,qemu/skiboot,open-power/skiboot,open-power/skiboot,qemu/skiboot,legoater/skiboot