[LPC4337] Fix RTC clock setting issue

- Fixed missing RTC clock intialization code
- Confirm to pass RTC test case (MBED_16)
[LPC4337] Remove init variable for RTC

- Remove static variable for initialization check
- Add enabled flag check for RTC control register
Merge pull request #815 from toyowata/master

Targets: LPC4337 - Fix RTC clock setting issue