Documentation efi-stub.rst: fix arm64 EFI source location

arch/arm64/kernel/efi-entry.S has been moved to
drivers/firmware/efi/libstub/arm64-entry.S by commit
v6.1-rc4-6-g4ef806096bdb and to
drivers/firmware/efi/libstub/arm64.c by commit
v6.2-rc3-6-g617861703830

Signed-off-by: Mikko Rapeli <mikko.rapeli@linaro.org>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20230904113214.41470215-1-mikko.rapeli@linaro.org>