Fix CC2538 random_init lockup

Contiki sometimes fails to boot correctly and locks up in
random_init()

This problem only manifests itself for specific versions
of the arm-gcc toolchain and then again only for specific
levels of optimisation (-Os vs -O2, depending on the
value of the SMALL make variable)

The lockup is caused when we write an RFCORE XREG before
the RF clock ungating has taken effect, which in turn
only occurs depending on the assembly generated for those
two instructions:

  REG(SYS_CTRL_RCGCRFC) = 1;

  REG(RFCORE_XREG_FRMCTRL0) = 0x00000008;

This commit makes the RNG wait for the ungating to take
effect before attempting to write the register