ARM: remove CONFIG_ARM926EJS defines

CONFIG_CPU_ARM926EJS was introduced into Kconfig by commit 2e07c249
(kconfig: arm: introduce symbol for ARM CPUs).

This commit removes all the defines of CONFIG_ARM926EJS and replaces
the only reference in arch/arm/lib/cache.c with CONFIG_CPU_ARM926EJS.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>