arm/efi: Improve performance requesting filesystem handle

arm/efi: Improve performance requesting filesystem handle

Currently, the code used to handle and possibly load from the filesystem
modules defined in the DT is allocating and closing the filesystem handle
for each module to be loaded.

To improve the performance, the filesystem handle pointer is passed
through the call stack, requested when it's needed only once and closed
if it was allocated.

Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>