misc: Add support for nvmem cells

misc: Add support for nvmem cells

This adds support for "nvmem cells" as seen in Linux. The nvmem device
class in Linux is used for various assorted ROMs and EEPROMs. In this
sense, it is similar to UCLASS_MISC, but also includes
UCLASS_I2C_EEPROM, UCLASS_RTC, and UCLASS_MTD. New drivers corresponding
to a Linux-style nvmem device should be implemented as one of the
previously-mentioned uclasses. The nvmem API acts as a compatibility
layer to adapt the (slightly different) APIs of these uclasses. It also
handles the lookup of nvmem cells.

While nvmem devices can be accessed directly, they are most often used
by reading/writing contiguous values called "cells". Cells typically
hold information like calibration, versions, or configuration (such as
mac addresses).

nvmem devices can specify "cells" in their device tree:

	qfprom: eeprom@700000 {
		#address-cells = <1>;
		#size-cells = <1>;
		reg = <0x00700000 0x100000>;

		/* ... */

		tsens_calibration: calib@404 {
			reg = <0x404 0x10>;
		};
	};

which can then be referenced like:

	tsens {
		/* ... */
		nvmem-cells = <&tsens_calibration>;
		nvmem-cell-names = "calibration";
	};

The tsens driver could then read the calibration value like:

	struct nvmem_cell cal_cell;
	u8 cal[16];
	nvmem_cell_get_by_name(dev, "calibration", &cal_cell);
	nvmem_cell_read(&cal_cell, cal, sizeof(cal));

Because nvmem devices are not all of the same uclass, supported uclasses
must register a nvmem_interface struct. This allows CONFIG_NVMEM to be
enabled without depending on specific uclasses. At the moment,
nvmem_interface is very bare-bones, and assumes that no initialization
is necessary. However, this could be amended in the future.

Although I2C_EEPROM and MISC are quite similar (and could likely be
unified), they present different read/write function signatures. To
abstract over this, NVMEM uses the same read/write signature as Linux.
In particular, short read/writes are not allowed, which is allowed by
MISC.

The functionality implemented by nvmem cells is very similar to that
provided by i2c_eeprom_partition. "fixed-partition"s for eeproms does
not seem to have made its way into Linux or into any device tree other
than sandbox. It is possible that with the introduction of this API it
would be possible to remove it.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>