xilinx: Enable NVMEM framework for all platforms

xilinx: Enable NVMEM framework for all platforms

Boards which have for example MAC address in eeprom but not in Xilinx
format (legacy or FRU) could reference it via nvmem cells.
For example:

&gem0 {
	nvmem-cells = <&mac>;
	nvmem-cell-names = "mac-address";
};

&eeprom {
	#address-cells = <1>;
	#size-cells = <1>;
	mac: mac-address@f0 {
		reg = <0xf0 6>;
	};
};

For getting it work above DT changes are required but also CONFIG_NVMEM
should be enabled. That's why enable it by default in generic defconfigs
to be able to use it directly by changing DT only.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/9c8ee7a4c7a16367438a92a4c9581bac9d968f84.1712815454.git.michal.simek@amd.com