rtc: rtc-ds1307: enable support for mcp794xx as a wakeup source without IRQ

This patch extends the fixes for ds1337, ds1339, ds3231 in commit
8bc2a407 ("rtc: ds1307: add support for the DT property
'wakeup-source'") to mcp794xx devices, so that those parts can similarly be
used as a wakeup source without an IRQ to the processor.

Tested on Raspberry Pi ZeroW with MCP79400.

Signed-off-by: David Lowe <dave-lowe@ntlworld.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>