ARM: 8313/1: Use read_cpuid_ext() macro instead of inline asm

Replace inline asm statement in __get_cpu_architecture() with equivalent
macro invocation, i.e. read_cpuid_ext(CPUID_EXT_MMFR0);

As an added bonus, this squashes a potential bug, described by Paul
Walmsley in commit 067e710b ("ARM: 7801/1: prevent gcc 4.5 from
reordering extended CP15 reads above is_smp() test").

Signed-off-by: Marc Gonzalez <marc_gonzalez@sigmadesigns.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>