ram: sifive: Fix compiler warnings for 32-bit

priv->info.size is of type 'size_t' but the length modifier is l.
Fix this by casting priv->info.size. Note 'z' cannot be used as
the modifier as SPL does not support that.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Rick Chen <rick@andestech.com>
Reviewed-by: Pragnesh Patel <pragnesh.patel@openfive.com>