ARM: 7753/1: map_init_section flushes incorrect pmd

This bug was introduced in commit e651eab0.
Some v4/v5 platforms failed to boot due to this.

Signed-off-by: Po-Yu Chuang <ratbert.chuang@gmail.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>