arm: TI am335x ehrpwm fix bitmask
Bitfield HSPCLKDIV and CLKDIV are 3 bit wide.
Chapter 15.2.4.1 https://www.ti.com/lit/ug/spruh73q/spruh73q.pdf
Approved by: manu (mentor)
Reported by: Stefan Forsberg
Differential revision: https://reviews.freebsd.org/D30746