cmd: Add fsinfo command

Add the fsinfo command to display filesystem statistics including block
size, total blocks, used blocks, and free blocks. Both raw byte counts
and human-readable sizes are shown.

Example output:
  => fsinfo mmc 0:1
  Block size: 4096 bytes
  Total blocks: 16384 (67108864 bytes, 64 MiB)
  Used blocks: 2065 (8458240 bytes, 8.1 MiB)
  Free blocks: 14319 (58650624 bytes, 55.9 MiB)

Co-developed-by: Claude Opus 4.5 <noreply@anthropic.com>
Signed-off-by: Simon Glass <simon.glass@canonical.com>

doc/usage/cmd/fsinfo.rst

@@ -0,0 +1,53 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+.. index::
+   single: fsinfo (command)
+
+fsinfo command
+==============
+
+Synopsis
+--------
+
+::
+
+    fsinfo <interface> <dev[:part]>
+
+Description
+-----------
+
+The fsinfo command displays filesystem statistics for a partition including
+block size, total blocks, used blocks, and free blocks. Both raw byte counts
+and human-readable sizes are shown.
+
+interface
+    interface for accessing the block device (mmc, sata, scsi, usb, ....)
+
+dev
+    device number
+
+part
+    partition number, defaults to 1
+
+Example
+-------
+
+::
+
+    => fsinfo mmc 0:1
+    Block size: 4096 bytes
+    Total blocks: 16384 (67108864 bytes, 64 MiB)
+    Used blocks: 2065 (8458240 bytes, 8.1 MiB)
+    Free blocks: 14319 (58650624 bytes, 55.9 MiB)
+
+Configuration
+-------------
+
+The fsinfo command is only available if CONFIG_CMD_FS_GENERIC=y.
+
+Return value
+------------
+
+The return value $? is set to 0 (true) if the command succeeded and to 1
+(false) otherwise. If the filesystem does not support statfs, an error
+message is displayed.