ARM: ensure C page table setup code follows assembly code

Fix a long standing bug where, for ARMv6+, we don't fully ensure that
the C code sets the same cache policy as the assembly code.  This was
introduced partially by commit 11179d8c

 ([ARM] 4497/1: Only allow
safe cache configurations on ARMv6 and later) and also by adding SMP
support.

This patch sets the default cache policy based on the flags used by the
assembly code, and then ensures that when a cache policy command line
argument is used, we verify that on ARMv6, it matches the initial setup.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>