lmb: Rename free_mem to available_mem

lmb: Rename free_mem to available_mem

free_mem is a misnomer. We never update it with the free memory for
LMB. Instead, it describes all available memory and is checked against
used_mem to decide whether an area is free or not.

So let's rename this field to better match its usage.

Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>
Tested-by: Sam Protsenko <semen.protsenko@linaro.org>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>