arm: qemu: enable LPAE on 32-bit

QEMU's mach-virt machine only supports selecting CPU models that
implement the virtualization extensions, and are therefore guaranteed
to support LPAE as well.

Initially, QEMU would not allow emulating these CPUs running in HYP
mode (or EL2, for AArch64), but today, it also contains a complete
implementation of the virtualization extensions themselves.

This means we could be running U-Boot in HYP mode, in which case the
LPAE long descriptor page table format is the only format that is
supported. If we are not running in HYP mode, we can use either.

So let's enable CONFIG_ARMV7_LPAE for qemu_arm_defconfig so that we
get the best support for running with the MMU and caches enabled at
any privilege level.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Heinrich Schuchardt <xypron.glpk@gmx.de>