nimble/controller: Add LP clock sysval to configure BSP
nimble/controller: Add LP clock sysval to configure BSP

Controller package defines new sysval (BLE_LP_CLOCK) which is then used
to configure ticks and timers accordingly.

X-Original-Commit: 0f20fa272a88c8af7ae02d94475f618148cba3a6

apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble