TIPWMSS is present on TI's DRA7x and AM437x SoCs. Enable its usage.

Instead of adding each SoC individually, use the more generic symbol
ARCH_OMAP2PLUS instead.
Signed-off-by: Vignesh R <vigneshr@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>