ARM: 8744/1: don't discard memblock for kexec

Discarding the memblock arrays usually works, but causes problems
with kexec, as pointed out by this kbuild warning:

WARNING: vmlinux.o(.text+0x7c60): Section mismatch in reference from the function machine_kexec_prepare() to the function .meminit.text:memblock_is_region_memory()

This lets us keep the memblock structures around whenever kexec
is enabled, but otherwise still drops them.

Fixes: cf1b0990 ("ARM: 8693/1: discard memblock arrays when possible")
Acked-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>