doc: samples: bluetooth: fast_pair: locator_tag: DFU for nRF54LM20

Clarifies image signature algorithm and the public key storage solution
for the nrf54lm20dk/nrf54lm20a/cpuapp target.

Jira: NCSDK-34491

Signed-off-by: Aleksander Strzebonski <[email protected]>

Author: alstrzebonski

samples/bluetooth/fast_pair/locator_tag/README.rst

@@ -232,7 +232,10 @@ The configuration of the signature algorithm and the public key storage solution
 | ED25519                        | * ``nrf54l15dk/nrf54l05/cpuapp`` (only ``release`` configuration) | Key Management Unit (KMU) | HW-accelerated (CRACEN),  |
 |                                | * ``nrf54l15dk/nrf54l10/cpuapp``                                  |                           | Signature derived from    |
 |                                | * ``nrf54l15dk/nrf54l15/cpuapp``                                  |                           | image (pure)              |
-|                                | * ``nrf54lm20dk/nrf54lm20a/cpuapp``                               |                           |                           |
++--------------------------------+-------------------------------------------------------------------+---------------------------+---------------------------+
+| ED25519                        | * ``nrf54lm20dk/nrf54lm20a/cpuapp``                               | Bootloader partition      | SW calculation,           |
+|                                |                                                                   |                           | Signature derived from    |
+|                                |                                                                   |                           | image (pure)              |
 +--------------------------------+-------------------------------------------------------------------+---------------------------+---------------------------+
 
 .. note::