mtd: spi: Support auto merge

In order to enable dual NOR flash users to experience double the
capacity and avoid frequent switching between two NOR flash devices,
the two NOR flash devices are virtualized into one device, which
I name it auto_merge tech.

Change-Id: I5edd7cde0481b1de6a35fce7ac67068889ff5ffe
Signed-off-by: Jon Lin <jon.lin@rock-chips.com>