cci: Wait before reading status register

The functions cci_enable_snoop_dvm_reqs and cci_disable_snoop_dvm_reqs write
in the SNOOP_CTRL_REGISTER of the slave interface and it polls the status
register to be sure that the operation is finished before leaving the
functions. If the write in SNOOP_CTRL_REGISTER is reordered after the first
read in the status register then these functions can finish before
enabling/disabling snoops and DVM messages.

The CCI500 TRM specifies:

	Wait for the completion of the write to the Snoop Control Register
	before testing the change_pending bit.

Change-Id: Idc7685963f412be1c16bcd3c6e3cca826e2fdf38
Signed-off-by: Roberto Vargas <roberto.vargas@arm.com>