ptp: dp83640: don't define PAGE0

Building dp83640.c on arch/parisc/ produces a build warning for
PAGE0 being redefined. Since the macro is not used in the dp83640
driver, just make it a comment for documentation purposes.

In file included from ../drivers/net/phy/dp83640.c:23:
../drivers/net/phy/dp83640_reg.h:8: warning: "PAGE0" redefined
    8 | #define PAGE0                     0x0000
                 from ../drivers/net/phy/dp83640.c:11:
../arch/parisc/include/asm/page.h:187: note: this is the location of the previous definition
  187 | #define PAGE0   ((struct zeropage *)__PAGE_OFFSET)

Fixes: cb646e2b ("ptp: Added a clock driver for the National Semiconductor PHYTER.")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Richard Cochran <richard.cochran@omicron.at>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Heiner Kallweit <hkallweit1@gmail.com>
Cc: Russell King <linux@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20210913220605.19682-1-rdunlap@infradead.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>