BL2U: Fix ARM platform timer initilization

This issue was detected when testing FWU on Juno. The Timer
`timer_ops` was not being initialized before being used by
the SDS driver on Juno. This patch adds the call to
`generic_delay_timer_init()` during bl2u_early_platform_setup().
This is done generically for all ARM platforms because the
cost involved is minimal.

Change-Id: I349cf0bd1db68406eb2298b65f9c729f792cabdc
Signed-off-by: Soby Mathew <soby.mathew@arm.com>