ARM: uniphier: increase CONFIG_SYS_MONITOR_LEN to 2MB

I increased the maximum U-Boot proper size from time to time, but
configs/uniphier_v7_defconfig hit the current limit 832KB.

Some historical info:

In the initial support, the max size was 512MB.

Commit 58d70227 ("ARM: uniphier: increase CONFIG_SYS_MONITOR_LEN")
increased it to 576KB, and commit 3ce5b1a8 ("ARM: uniphier: move
SPL stack address") moved the SPL stack location to avoid the memory
map conflict. It was the solution to increase the size without changing
the NOR boot image map.

commit 1a4bd3a0 ("ARM: uniphier: increase CONFIG_SYS_MONITOR_LEN
again") ended up with increasing the max size again, breaking the NOR
boot image map. The limit was set to 832KB, otherwise the SPL stack
would overwrite the U-Boot proper image:
 CONFIG_SPL_STACK - CONFIG_SYS_UBOOT_BASE + sizeof(struct image_header) = 0xd0000

To increase CONFIG_SYS_MONITOR_LEN even more, the SPL stack must be
moved somewhere. I put it back to the original location prior to
commit 3ce5b1a8.

With this change, there is no more practical size limit.

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