mtd: devices: elm: Low power transition support

ELM is used for locating bit-flip errors in when using BCH ECC scheme.
This patch adds suspend/resume support for leaf level ELM driver,
And also provides ELM register context save & restore support, so that
configurations are preserved across hardware power-off/on transitions.

Signed-off-by: Philip Avinash <avinashphilip@ti.com>
Signed-off-by: Pekon Gupta <pekon@ti.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>