[IA64] fix "allnoconfig" build

Link errors:
arch/ia64/kernel/built-in.o: In function `arch_setup_dmar_msi':
(.text+0x35972): undefined reference to `dmar_msi_write'
... and more ...

because allnoconfig has CONFIG_DMAR=y due to the "select DMAR"
in arch/ia64/Kconfig under config IA64_GENERIC.

Drop that select, but add CONFIG_DMAR=y to generic_defconfig so
we keep testbuilding the DMAR code.

Signed-off-by: Tony Luck <tony.luck@intel.com>