kconfig: arm: introduce symbol for ARM CPUs

This commit introduces a Kconfig symbol for each ARM CPU:
CPU_ARM720T, CPU_ARM920T, CPU_ARM926EJS, CPU_ARM946ES, CPU_ARM1136,
CPU_ARM1176, CPU_V7, CPU_PXA, CPU_SA1100.
Also, it adds the CPU feature Kconfig symbol HAS_VBAR which is selected
for CPU_ARM1176 and CPU_V7.

For each target, the corresponding CPU is selected and the definition of
SYS_CPU in the corresponding Kconfig file is removed.

Also, it removes redundant "string" type in some Kconfig files.

Signed-off-by: Georges Savoundararadj <savoundg@gmail.com>
Acked-by: Albert ARIBAUD <albert.u.boot@aribaud.net>
Cc: Masahiro Yamada <yamada.m@jp.panasonic.com>