of: Add support for reading a u32 from a multi-value property.

This patch adds an of_property_read_u32_index() function to allow
reading a single indexed u32 value from a property containing multiple
u32 values.

Signed-off-by: Tony Prisk <linux@prisktech.co.nz>
Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
pinctrl: bcm2835: make use of of_property_read_u32_index()

Use the new standard API of_property_read_u32_index() instead of open-
coding it.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
---
Note: This depends on the proposed patch "of: Add support for reading
a u32 from a multi-value property" by Tony Prisk.

BTW, I realized why I didn't create that standard API, but wrote custom
prop_u32() instead; the code has already looked up the properties, and
validated their length, so prop_u32() can simply index into the property
data, whereas of_property_read_u32_index() needs to go search for the
property and re-validate it every time, so there's a bunch more overhead.
It also means duplicating the property name, although I could use a
define for that. I'm not entirely convinced that using this standard API
is a win in this case. LinusW, Tony, what do you think?