Kconfig: arm64: Update dependancy for CONFIG_FREE_PAGES_RDONLY

The kernel_map_pages function is compiled only when
CONFIG_PAGE_POISONING is enabled, however the stub
fuction is added when CONFIG_DEBUG_PAGEALLOC=n, which
causes a compilation error when CONFIG_DEBUG_PAGEALLOC
is disabled and CONFIG_FREE_PAGES_RDONLY is enabled.

Enable CONFIG_DEBUG_PAGEALLOC when CONFIG_FREE_PAGES_RDONLY
is enabled.

Change-Id: I576886982768d85e112d7f9e7d92d588a735acc1
Signed-off-by: Neeti Desai <neetid@codeaurora.org>