misc: sram: extend usage of reserved partitions

This change adds functionality to operate on reserved SRAM partitions
described in device tree file. Two partition properties are added,
"pool" and "export", the first one allows to share a specific partition
for usage by a kernel consumer in the same manner as it is done for
the whole SRAM device, and "export" property provides access to some
SRAM area from userspace over sysfs interface. Practically it is
possible to specify both properties for an SRAM partition, however
simultaneous access from a kernel consumer and from userspace is not
serialized, but still the combination may be useful for debugging
purpose.

The change opens the following scenarios of SRAM usage:
* updates in a particular SRAM area specified by offset and size are
  done by bootloader, then this information is utilized by the kernel,
* a particular SRAM area is rw accessed from userspace, the stored
  data is persistent on soft reboots,
* a device driver secures SRAM area for its purposes,
* etc.

Note, strictly speaking the added optional properties describe policy
of SRAM usage, rather than hardware, but here the policy mostly
resembles flash partitions in devicetree, which is undoubtedly
a very popular option but it does not describe hardware.

Signed-off-by: Vladimir Zapolskiy <vladimir_zapolskiy@mentor.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>