mlxbf-bootctl: Add sysfs file for BlueField boot log

mlxbf-bootctl: Add sysfs file for BlueField boot log

This commit adds sysfs interface to be used to write into the
boot log which is 1KB HW buffer on BlueField SoC. The same log
buffer is also used by firmware code like ATF/UEFI, and can be
displayed by userspace tools or from external host via USB/PCIe.

Signed-off-by: Liming Sun <limings@nvidia.com>
Reviewed-by: Vadim Pasternak <vadimp@nvidia.com>
Link: https://lore.kernel.org/r/20230511144924.171585-1-limings@nvidia.com
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>