vexpress/aemv8a: drop CONFIG_ARMV8_SWITCH_TO_EL1

To support KVM, we need to drop at EL2 and not EL1 before we boot Linux
kernel. This causes issues on platform with VHE and secondaries booting
at EL2 via TF-A PSCI CPU_ON call.

Cc: Ryan Harkin <ryan.harkin@linaro.org>
Cc: Liviu Dudau <liviu.dudau@foss.arm.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: David Feng <fenghua@phytium.com.cn>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>