Kconfig: arm: 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: I89f3dc73d4031b560e0595529498d71e77e83720
Signed-off-by: Neeti Desai <neetid@codeaurora.org>