eeprom: Pull out CONFIG_SYS_EEPROM_PAGE_WRITE_BITS

eeprom: Pull out CONFIG_SYS_EEPROM_PAGE_WRITE_BITS

Implement default value of 8 for this macro and pull out all of
this macro out of the code. The default value of 8 actually does
implement exactly the same behavior as the previous code which
was in the #else clause of the ifdef.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>
Cc: Heiko Schocher <hs@denx.de>
Reviewed-by: Heiko Schocher <hs@denx.de>