power_supply: add eeprom dump file to olpc_battery's sysfs

This allows you to dump 0x60 bytes from the battery's EEPROM (starting at
address 0x20).  Note that it does an EC command for each byte, so it's pretty
slow.  OTOH, if you want to grab just a single byte from somewhere in the
EEPROM, you can do something like:

dd bs=1 count=1 skip=16 if=/sys/class/power_supply/olpc-battery/eeprom | od -x

Userspace battery collection/logging information needs this.

Signed-off-by: Andres Salomon <dilinger@debian.org>
Cc: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>