docs(cert-create): add key size options for ecdsa

Adding the possible key sizes for the ecdsa key algorithm.

Change-Id: I58947bc749fed911766a1462a0c2ba520b8f7c69
Signed-off-by: Lauren Wehrmeister <lauren.wehrmeister@arm.com>
diff --git a/docs/getting_started/build-options.rst b/docs/getting_started/build-options.rst
index cf8002d..cd70a22 100644
--- a/docs/getting_started/build-options.rst
+++ b/docs/getting_started/build-options.rst
@@ -692,7 +692,7 @@
    +===========================+====================================+
    |           rsa             | 1024 , 2048 (default), 3072, 4096* |
    +---------------------------+------------------------------------+
-   |          ecdsa            |            unavailable             |
+   |          ecdsa            |         256 (default), 384         |
    +---------------------------+------------------------------------+
    |  ecdsa-brainpool-regular  |            unavailable             |
    +---------------------------+------------------------------------+