pwm: lpss: Fix build failure on PowerPC

An x86 build seems to pull in the linux/io.h include indirectly. On
PowerPC that doesn't happen and the build breaks due to the readl() and
writel() functions not being declared. Fix this by explicitly including
linux/io.h.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>