ARM: Drop unreferenced CONFIG_* defines named after SoCs

The following config symbols are only defined once and never referenced
anywhere else:

CONFIG_ARM926EJS
CONFIG_CPUAT91
CONFIG_EXYNOS5800
CONFIG_SYS_CORTEX_R4

Most of them are config symbols named after the respective SoCs which
seems to have been a standard practice at some point.

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>