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: cb646e2b02b2 ("ptp: Added a clock driver for the National Semiconductor PHYTER.")
Signed-off-by: Randy Dunlap <[e-mail]>
Reported-by: Geert Uytterhoeven <[e-mail]>
Cc: Richard Cochran <[e-mail]>
Cc: John Stultz <[e-mail]>
Cc: Heiner Kallweit <[e-mail]>
Cc: Russell King <[e-mail]>
Reviewed-by: Andrew Lunn <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/phy/dp83640_reg.h

···
 #ifndef HAVE_DP83640_REGISTERS
 #define HAVE_DP83640_REGISTERS
 
-#define PAGE0                     0x0000
+/* #define PAGE0                  0x0000 */
 #define PHYCR2                    0x001c /* PHY Control Register 2 */
 
 #define PAGE4                     0x0004