ath79: routerboard-92x: ensure explicit bios size in DTSI

ath79: routerboard-92x: ensure explicit bios size in DTSI

The missing "size" property was acceptable in the context of a single
DTS as the underlying device is known to have a 64KB flash, and thus
the bios partition fit exactly between the preceding and following ones.

However as this block has moved in a DTSI, for the sake of clarity and
explicitness the size property is added to ensure that if the flash
happens to be larger than expected, the bios partition remains properly
sized.

Suggested-by: Thibaut VARÈNE <hacks@slashdirt.org>
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>