nvram: increase NVRAM size to 64 KiB

nvram: increase NVRAM size to 64 KiB

For years Broadcom devices use 64 KiB NVRAM partition size and some of
them indeed have it filled in more than 50%. This change allows handling
whole NVRAM e.g. on Netgear WNDR4500 and Netgear R8000.

The same fix was applied to kernel in upstream commit 6ab7c29.

Reported-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45566 3c298f89-4303-0410-b956-a3cf2f4a3e73