ARM: vexpress: Check master site in daughterboard's sysctl operations

With recent enough motherboard firmware, core tile can be fitted
in either of the two daughterboard sites. The non-DT tile code for
V2P-CA9 did not check that when configuring DVI output nor setting
CLCD pixel clock.

Fixed now, providing "get master site" API in motherboard's code.

Signed-off-by: Pawel Moll <pawel.moll@arm.com>