block: m25p80: Introduce die erase command

Modern big flash NOR devices consist of more than one die.
Some of them do not support chip erase and instead have a die
erase command that can erase one die only. This commit adds
support for defining the number of dies in the chip, and adds
support for die erase command.

The NOR flash model is not strict, so no option to
disable chip erase has been added.

Signed-off-by: Marcin Krzeminski <mar.krzeminski@gmail.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Message-id: 20170108083854.5006-3-mar.krzeminski@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>