arm64: kernel: remove non-legit DT warnings when booting using ACPI

Since both CONFIG_ACPI and CONFIG_OF are enabled when booting using ACPI
tables on ARM64 platforms, we get few device tree warnings which are not
valid for ACPI boot. We can use of_have_populated_dt to check if the
device tree is populated or not before throwing out those errors.

This patch uses of_have_populated_dt to remove non legitimate device
tree warning when booting using ACPI tables.

Cc: Lorenzo Pieralisi <Lorenzo.Pieralisi@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>