mfd: sec-core: Remove unused s2mpu02-rtc and s2mpu02-clk children

The commit 54e8827d5f0e ("mfd: sec-core: Add support for S2MPU02
device") added new MFD child devices for S2MPU02: RTC and clock
provider (the clock provider with new compatible). However support for
these devices was not added to existing drivers (rtc-s5m, clk-s2mps11).
New drivers were not submitted neither.

This means that the name of children devices is completely unused. The
"samsung,s2mpu02-clk" compatible remains undocumented so it is unclear
what is provided by that compatible.

Clean up this by removing unused child devices and undocumented
compatible.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>