mt76: mt7915: add support for flash mode

Add support for getting rf values from flash.
This is used for some test purposes and products.
If the mtd partition is configured in dts, driver will read from flash
to init eeprom command; if not, still init it with efuse's values.

An example:
&slot0 {
	mt7915@0,0 {
		reg = <0x0000 0 0 0 0>;
		device_type = "pci";
		mediatek,mtd-eeprom = <&factory 0x0000>;
	};
};

Acked-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>