mlxbf-bootctl: Support sysfs entries for RTC battery status

mlxbf-bootctl: Support sysfs entries for RTC battery status

This patch extends the mlxbf-bootctl driver's sysfs entries
to support read access for the board's RTC battery status.
A successful read from this attribute returns the status of
the board's RTC battery. The RTC battery status register is
also cleared upon successful read operation.

Signed-off-by: Xiangrong Li <xiangrongl@nvidia.com>
Reviewed-by: David Thompson <davthompson@nvidia.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20250124144655.48564-1-xiangrongl@nvidia.com
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>