alpha: Reorganize rtc handling

Discontinue use of GENERIC_CMOS_UPDATE; rely on the RTC subsystem.

The marvel platform requires that the rtc only be touched from the
boot cpu.  This had been partially implemented with hooks for
get/set_rtc_time, but read/update_persistent_clock were not handled.
Move the hooks from the machine_vec to a special rtc_class_ops struct.

We had read_persistent_clock managing the epoch against which the
rtc hw is based, but this didn't apply to get_rtc_time or set_rtc_time.
This resulted in incorrect values when hwclock(8) gets involved.

Allow the epoch to be set from the kernel command-line, overriding
the autodetection, which is doomed to fail in 2020.  Further, by
implementing the rtc ioctl function, we can expose this epoch to
userland.

Elide the alarm functions that RTC_DRV_CMOS implements.  This was
highly questionable on Alpha, since the interrupt is used by the
system timer.

Signed-off-by: Richard Henderson <[email protected]>

drivers/rtc/Kconfig

@@ -626,7 +626,7 @@ comment "Platform RTC drivers"
 
 config RTC_DRV_CMOS
 	tristate "PC-style 'CMOS'"
-	depends on X86 || ALPHA || ARM || M32R || ATARI || PPC || MIPS || SPARC64
+	depends on X86 || ARM || M32R || ATARI || PPC || MIPS || SPARC64
 	default y if X86
 	help
 	  Say "yes" here to get direct support for the real time clock
@@ -643,6 +643,14 @@ config RTC_DRV_CMOS
 	  This driver can also be built as a module. If so, the module
 	  will be called rtc-cmos.
 
+config RTC_DRV_ALPHA
+	bool "Alpha PC-style CMOS"
+	depends on ALPHA
+	default y
+	help
+	  Direct support for the real-time clock found on every Alpha
+	  system, specifically MC146818 compatibles.  If in doubt, say Y.
+
 config RTC_DRV_VRTC
 	tristate "Virtual RTC for Intel MID platforms"
 	depends on X86_INTEL_MID