efi_loader: Increase default variable store size to 128K

In commit 9fd3f881 ("efi_loader: Increase default variable store size to 64KiB")
Alper has a detailed explanation of why the size needs to be bumped to at
least 64K.  However enabling Secure boot, writing db, KEK, PK etc keys
will further increase the size so bump it to 128K.

It's worth noting that when U-Boot stores the EFI variables in an RPMB the
available storage is defined statically in StandAloneMM at build time.
The U-Boot code is detecting the available true size on the fly during
writes. When StandAloneMM is present this size defines the reserved
memory U-Boot can use to copy any runtime variables, before booting an
OS.

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>