ARM: at91: sama5d2: Wrap cpu detection to fix macb driver

ARM: at91: sama5d2: Wrap cpu detection to fix macb driver

When introducing the SAMA5D27 SoCs, the SAMA5D2 series got an additional
chip id. The check if the cpu is sama5d2 was changed from a preprocessor
definition (inlining a call to 'get_chip_id()') to a C function,
probably to not call get_chip_id twice?

That however broke a check in the macb ethernet driver. That driver is
more generic and also used for other platforms. I suppose this solution
was implemented to use it in 'gem_is_gigabit_capable()', without having
to stricly depend on the at91 platform:

	#ifndef cpu_is_sama5d2
	#define cpu_is_sama5d2() 0
	#endif

That only works as long as cpu_is_sama5d2 is a preprocessor definition.
(The same is still true for sama5d4 by the way.) So this is a straight
forward fix for the workaround.

The not working check on the SAMA5D2 CPU lead to an issue on a custom
board with a LAN8720A ethernet phy connected to the SoC:

	=> dhcp
	ethernet@f8008000: PHY present at 1
	ethernet@f8008000: Starting autonegotiation...
	ethernet@f8008000: Autonegotiation complete
	ethernet@f8008000: link up, 1000Mbps full-duplex (lpa: 0xffff)
	BOOTP broadcast 1
	BOOTP broadcast 2
	BOOTP broadcast 3
	BOOTP broadcast 4
	BOOTP broadcast 5
	BOOTP broadcast 6
	BOOTP broadcast 7
	BOOTP broadcast 8
	BOOTP broadcast 9
	BOOTP broadcast 10
	BOOTP broadcast 11
	BOOTP broadcast 12
	BOOTP broadcast 13
	BOOTP broadcast 14
	BOOTP broadcast 15
	BOOTP broadcast 16
	BOOTP broadcast 17

	Retry time exceeded; starting again

Notice the wrong reported link speed, although both SoC and phy only
support 100 MBit/s!

The real issue on reliably detecting the features of that cadence
ethernet mac IP block, is probably more complicated, though.

Fixes: 245cbc583d ("ARM: at91: Get the Chip ID of SAMA5D2 SiP")
Signed-off-by: Alexander Dahl <ada@thorsis.com>