habanalabs: revise and document use of boot status flags

The boot status flag "SRAM available" can be set by f/w Linux (in the
general case) or by f/w uboot (in some specific debug scenario) but
never by f/w preboot.

Hence, when polling the boot status flags in the preboot stage we do not
want to poll on "SRAM Avialable".

The special case in which uboot set this flag is when we are running
special debug scenario without Linux. In this case, at some point during
the boot, the uboot relocates its code to the DRAM and then set the
specified flag.

Signed-off-by: Ohad Sharabi <osharabi@habana.ai>
Reviewed-by: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Oded Gabbay <ogabbay@kernel.org>