boards: nrf52_blenano2: standardize scratch and storage partitions

Most of the other nRF52832 boards have the following settings for
scratch and storage partitions:

scratch_partition: partition@70000 {
	label = "image-scratch";
	reg = <0x00070000 0xa000>;
};

storage_partition: partition@7a000 {
	label = "storage";
	reg = <0x0007a000 0x00006000>;
};

Let's remove the snowflake settings in BLENano2 which coincidentally
are incorrect: storage overruns the flash area by 0x1000 and causes
a crash when enabling FCB storage.

Signed-off-by: Michael Scott <mike@foundries.io>