rtc: ep93xx: drop needless struct ep93xx_rtc::rtc member

rtc: ep93xx: drop needless struct ep93xx_rtc::rtc member

The memory pointed to by the ::rtc member is managed via devres, and
no code in this driver uses it past _probe().

We can drop it from the structure and just use a local temporary
variable, reducing runtime memory consumption by a few bytes.

Signed-off-by: André Draszik <andre.draszik@linaro.org>
Link: https://lore.kernel.org/r/20250304-rtc-cleanups-v2-5-d4689a71668c@linaro.org
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>