ARM: uniphier: change CONFIG_SPL_PAD_TO to 128KB

The Boot ROM supports authentication feature to prevent malformed
software from being run on products.  The signature is added at the
tail of the second stage loader (= SPL in U-boot terminology).

The size of the second stage loader was 64KB, and it was consistent
across SoCs.  The situation changed when LD20 SoC appeared; it loads
80KB second stage loader, and it is the only exception.

Currently, CONFIG_SPL_PAD_TO is set to 64KB and U-Boot proper is
loaded from the 64KB offset of non-volatile devices.  This means the
signature of LD20 SoC (located at 80KB offset) corrupts the U-Boot
proper image.

Let's move the U-Boot proper image to 128KB offset.  It uses 48KB
for nothing but padding, and we could actually locate the U-Boot
proper at 80KB offset.  However, the power of 2 generally seems a
better choice for the offset address.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>