pinctrl: sunxi: Make sunxi_pconf_group_set use sunxi_pconf_reg helper
The sunxi_pconf_reg helper introduced in the last patch gives us the
chance to rework sunxi_pconf_group_set to have it match the structure
of sunxi_pconf_(group_)get and make it easier to understand.
For each config to set, it:
1. checks if the parameter is supported.
2. checks if the argument is within limits.
3. converts argument to the register value.
4. writes to the register with spinlock held.
As a result the function now blocks unsupported config parameters,
instead of silently ignoring them.