Add EHRPWM and ECAP support build support for DAVINCI_DA8XX platforms.

Also, since DAVINCI platforms doesn't support TI-PWM-Subsystem module,
remove the select option for CONFIG_PWM_TIPWMSS.

Also, update CONFIG_PWM_TIPWMSS compiler directive appropriately in
pwm-tipwmss.h to fix the below compiler error upon removal of
CONFIG_PWM_TIPWMSS for DAVINCI platforms.

	drivers/pwm/pwm-tiecap.c: In function 'ecap_pwm_probe':
	drivers/pwm/pwm-tiecap.c:263:4: error: 'PWMSS_ECAPCLK_EN' undeclared
	(first use in this function)
	drivers/pwm/pwm-tiecap.c:263:4: note: each undeclared identifier
	is reported only once for each function it appears in
	drivers/pwm/pwm-tiecap.c:264:17: error: 'PWMSS_ECAPCLK_EN_ACK'
	undeclared (first use in this function)
	drivers/pwm/pwm-tiecap.c: In function 'ecap_pwm_remove':
	drivers/pwm/pwm-tiecap.c:291:49: error: 'PWMSS_ECAPCLK_STOP_REQ'
	undeclared (first use in this function)
	make[2]: *** [drivers/pwm/pwm-tiecap.o] Error 1
	make[1]: *** [drivers/pwm] Error 2
	make: *** [drivers] Error 2
Signed-off-by: NPhilip Avinash <avinashphilip@ti.com>
Signed-off-by: NThierry Reding <thierry.reding@avionic-design.de>

In some platforms (like am33xx), PWM sub modules (ECAP, EHRPWM, EQEP)
are integrated to PWM subsystem. These PWM submodules has resources
shared and only one register bit-field is provided to control
module/clock enable/disable, makes it difficult to handle common
resources from independent PWMSS submodule drivers.

So the solution here implemented in this patch is, to create driver for
PWMSS and take the role of parent driver for PWM submodules. PWMSS
parent driver enumerates all the child nodes under PWMSS module. Also
symbol "pwmss_submodule_state_change" exported to enable clock gating
for individual PWMSS submodules, and submodule drivers has to enable
clock gating from their drivers.

As this is only supported during DT boot, the parent/child relationship
is created and populated in DT execution flow. The only required change
is inside DTS file, making EHRPWM & ECAP as a child to PWMSS node.
Signed-off-by: NPhilip, Avinash <avinashphilip@ti.com>
Signed-off-by: NThierry Reding <thierry.reding@avionic-design.de>