mtd-utils: add optional offset parameter to flash_otp_dump

There are flashes which have gaps between OTP regions and flashes where
the regions don't start at 0 (for example the Winbond 25Q series, which
has three 256 bytes OTP regions starting at 0x1000, 0x2000 and 0x3000).
At the moment it is impossible to dump the OTP memory. Fix it by passing
an optional offset parameter.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>