ARM: s3c: Avoid naming clash of S3C24xx and S3C64xx timer setup

PWM timer initialization has two independent implementations - one for
S3C24xx and one for S3C64xx.  The naming however was always the same
and before also the declaration was shared.  This is confusing, error
prone and might cause issues when trying to build multiplatform kernel.

Suggested-by: Tomasz Figa <tomasz.figa@gmail.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200820204203.22328-1-krzk@kernel.org