asm-generic/vmlix.lds.h: simplify __mem{init,exit}* dependencies

Simplify the dependencies on __mem{init,exit}* (ACPI_HOTPLUG_MEMORY requires
MEMORY_HOTPLUG).

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>