irqchip/gic-v3: Add missing barrier to 32bit version of gic_read_iar()

Commit 1a1ebd5f ("irqchip/gic-v3: Make sure read from ICC_IAR1_EL1 is
visible on redestributor") fixed the missing barrier on arm64, but
forgot to update the 32bit counterpart, which has the same requirements.
Let's fix it.

Fixes: 1a1ebd5f

 ("irqchip/gic-v3: Make sure read from ICC_IAR1_EL1 is visible on redestributor")
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>