ucm: implement ValueDefaults.BootCardGroup and define use

We need a boot synchronization for multiple UCM cards where linking
is expected like AMD ACP or Intel AVS drivers. This method is
using a timestamp file which can be created and modified during
the boot process (e.g. from the alsactl tool).

The goal is to return a valid UCM configuration for standard
applications combining multiple ALSA cards into one UCM configuration
and cover the time window when all cards have not been probed yet.

Signed-off-by: Jaroslav Kysela <perex@perex.cz>

src/ucm/ucm_confdoc.h

@@ -462,6 +462,38 @@ boot).
 
 \image html ucm-seq-boot.svg
 
+#### Boot Synchronization (Syntax 8+)
+
+The *BootCardGroup* value in *ValueDefaults* allows multiple sound cards to coordinate
+their boot sequences. This value is detected at boot (alsactl/udev/systemd) time. Boot
+tools can provide boot synchronization information through a control element named
+'Boot' with 64-bit integer type. When present, the UCM library uses this control element
+to coordinate initialization timing.
+
+The 'Boot' control element contains:
+- **index 0**: Boot time in CLOCK_MONOTONIC_RAW (seconds)
+- **index 1**: Restore time in CLOCK_MONOTONIC_RAW (seconds)
+- **index 2**: Primary card number (identifies also group)
+
+The UCM open call waits until the boot timeout has passed or until restore state
+is notified through the synchronization Boot element. The timeout defaults to 30 seconds
+and can be customized using 'BootCardSyncTime' in 'ValueDefaults' (maximum 240 seconds).
+
+If the 'Boot' control element is not present, no boot synchronization is performed.
+
+Other cards in the group (primary card number is different) will have the "Linked"
+value set to "1", allowing UCM configuration files to detect and handle secondary
+cards appropriately.
+
+Example configuration:
+
+~~~{.html}
+ValueDefaults {
+  BootCardGroup "amd-acp"
+  BootCardSyncTime 10 # seconds
+}
+~~~
+
 ### Device volume
 
 It is expected that the applications handle the volume settings. It is not recommended