hw/arm/boot: Set PC correctly when loading AArch64 ELF files

The code in do_cpu_reset() correctly handled AArch64 CPUs
when running Linux kernels, but was missing code in the
branch of the if() that deals with loading ELF files.
Correctly jump to the ELF entry point on reset rather than
leaving the reset PC at zero.

Reported-by: Christopher Covington <cov@codeaurora.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Tested-by: Christopher Covington <cov@codeaurora.org>
Cc: qemu-stable@nongnu.org