Rename RAMB_RTC_DH_* flags to RTCREG_DH_*

[Rangi42]

Supersedes #81.

These flag bits apply to the value mapped into [rRTCREG], not the value of [rRAMB]. rRAMB is only involved because writing the RAMB_RTC_DH sentinel value to it maps the RTC DH flags into [rRTCREG].

[Rangi42]

An example from pokecrystal:

StartRTC:
	ld a, RAMG_SRAM_ENABLE
	ld [rRAMG], a
	call LatchClock
	ld a, RAMB_RTC_DH
	ld [rRAMB], a
	ld a, [rRTCREG]
	res B_RAMB_RTC_DH_HALT, a
	ld [rRTCREG], a
	call CloseSRAM
	ret

This would make more sense after a rename:

StartRTC:
	ld a, RAMG_SRAM_ENABLE
	ld [rRAMG], a
	call LatchClock
	ld a, RAMB_RTC_DH
	ld [rRAMB], a
	ld a, [rRTCREG]
	res B_RTCREG_DH_HALT, a
	ld [rRTCREG], a
	call CloseSRAM
	ret