ARM: 7869/1: remove unused XSCALE_PMU Kconfig param

This removes the XSCALE_PMU Kconfig param, which is defined
but no longer used in makefiles and source files.

Signed-off-by: Michael Opdenacker <michael.opdenacker@free-electrons.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>