rtc: make HPET_RTC_IRQ track HPET_EMULATE_RTC

More Kconfig tweaks related to the legacy PC RTC code:

 - Describe the legacy PC RTC driver as such ... it's never quite
   been clear that this driver is for PC RTCs, and now it's fair
   to call this the "legacy" driver.

 - Force it to understand about HPET stealing its IRQs ... kernel
   code does this always when HPET is in use, there should be no
   option for users to goof up the config.

This seems to fix kernel bugzilla #10729.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: Maxim Levitsky <maximlevitsky@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>