ARM: io: simplify ioremap* and iounmap definitions

We don't need to repeat the same definitions of the ioremap*(),
once in terms of __arch_ioremap() and again in terms of __arm_ioremap().
Instead, if the platform hasn't provided an __arch_ioremap, define
this to be __arm_ioremap, and only define the ioremap*() set using
__arch_ioremap.

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