ARM: 8408/1: Fix the secondary_startup function in Big Endian case

Since the commit "b2c3e38a

 ARM: redo TTBR setup code for LPAE",
the setup code had been reworked. As a result the secondary CPUs
failed to come online in Big Endian.

As explained by Russell, the new code expected the value in r4/r5 to
be the least significant 32bits in r4 and the most significant 32bits
in r5. However, in the secondary code, we load this using ldrd, which
on BE reverses that.

This patch swap r4/r5 after the ldrd. It is done using the xor
instructions in order to not use a temporary register.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>