rtc: switch wakealarm attribute to DEVICE_ATTR_RW

Instead of using older style DEVICE_ATTR for wakealarm attribute let's
switch to using DEVICE_ATTR_RW that ensures consistent across the kernel
permissions on the attribute.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>