ARM: 9148/1: handle CONFIG_CPU_ENDIAN_BE32 in arch/arm/kernel/head.S

My intel-ixp42x-welltech-epbx100 no longer boot since 4.14.
This is due to commit 463dbba4 ("ARM: 9104/2: Fix Keystone 2 kernel
mapping regression")
which forgot to handle CONFIG_CPU_ENDIAN_BE32 as possible BE config.

Suggested-by: Krzysztof Hałasa <khalasa@piap.pl>
Fixes: 463dbba4 ("ARM: 9104/2: Fix Keystone 2 kernel mapping regression")
Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>