plat: marvell: ap806: implement workaround for errata-id FE-4265711

plat: marvell: ap806: implement workaround for errata-id FE-4265711

ERRATA ID: FE-4265711 - Incorrect CNTVAL reading

CNTVAL reflects the global system counter value in binary format.
Due to this erratum, the CNTVAL value presented to the processor
may be incorrect for several clock cycles.

Workaround: Override the default value of AP Register Device General
control 20 [19:16] and AP Register Device General Control 21 [11:8]
to the value of 0x3.

Change-Id: I1705608d08acd9631ab98d6f7ceada34d6b8336f
Signed-off-by: Stefan Chulski <stefanc@marvell.com>
Signed-off-by: Marcin Wojtas <mw@semihalf.com>