soc: qcom: smem: Update size of legacy partition

Added free offset while assigning size of legacy
partition. Legacy partition size is equal to the
sum of free offset and available size.

Acquired a spinlock while reading free offset and
available size to avoid race conditions.

Change-Id: Iff74b303b8321c6a83271fc0490aaf3d9fea4651
Signed-off-by: Vivek Golani <vgolani@codeaurora.org>