spmi: pmic-arb: correct support for up to 512 APIDs

Version 3 of the SPMI PMIC arbiter supports up to 512 APIDs.
However, the APID value is currently stored in u8 variables
throughout the spmi-pmic-arb driver.  Change the type of these
variables to be u16 so that APIDs 256-511 are not truncated.
Also update the out_hwirq value packing in order to support
more than 8 bits for APIDs.

Change-Id: If2408bc4678b2e0f7e2366329be005cd652661cf
CRs-Fixed: 1045396
Signed-off-by: David Collins <collinsd@codeaurora.org>