[ARM] 3792/2: Fix description of ZBOOT_ROM_BSS

The documentation for the ZBOOT_ROM_BSS config option describes it as
"The base address of 64KiB of read/write memory in the target for the
ROM-able zImage..." In actuality, it requires more than 100 KiB of
space in addition to enough space to hold the decompressed kernel.
This patch fixes the description in the Kconfig file.

Signed-off-by: Dan Fandrich <dfandrich@intrinsyc.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>