RTC Kconfig tweax

Minor fix to the Kconfig for RTCs: don't display section headers for I2C or
SPI unless they're configured.  And depend on SPI_MASTER; having slave
support wouldn't help.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Acked-by: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>