rtc-lib: drop unused rtc_ydays ...

The patch fixes this compiler's warning:

    drivers/rtc/rtc-lib.c:23:29: warning: ‘rtc_ydays’ defined but not used [-Wunused-const-variable=]
     static const unsigned short rtc_ydays[2][13] = {
                                 ^~~~~~~~~

Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

rtc-lib: Check tm_wday for validity in rtc_valid_tm() ...

RTC drivers rely on rtc_valid_tm() in order to make sure that no bogus
values from uninitialized HW registers get passed to the uppper layers.

A somewhat contrived way to reproduce this problem with DS1307 RTC
would be to do the following:

> i2c_write -b <bus> -a <addr> -r 3 0x00
> hwclock

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

rtc-lib: import rtc_time_to_tm() from linux-3.15 ...

Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

Add a simple rtc framework ...

This patch adds a simple rtc framework
for reading and setting board's RTC time.

Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>